Background & Aims: Thiamine (vitamine B1) serves as a cofactor for several enzymes involved in carbohydrate catabolism such as pyrovate dehydrogenase and transketolase and alfa-ketoglutarate. thiamine and related vitamine may improve the outcome in children with type 1 diabetes mellitus. Thiamine is absorbed efficiently in the gastrointestinal tract. adequate thiamine intake can be achived with a a varied diet that includes meat and enriched or whole _grain cereals. thiamine is water soluble and heat labile. The requirement of thiamine is increased when carbohydrates are taken in large amounts and during periods of increased metabolism, suc as diseases. thiamine deficiency is common in children with diabetic ketoACIDOSIS (DKA) and treatment of diabetic ketoACIDOSIS may exacerbate this deficiency. To prevent the complications of thiamine deficiency, ketoACIDOSIS caused by diabetes must first be eliminated and then, if metabolic ACIDOSIS persists, other factors affecting this process must be addressed. Due to the importance of the subject and the limited studies that have been done to measure the serum level of thiamine, especially in children and adolescents with diabetes, we found the serum level of this vitamin in two groups of children and adolescents with diabetic ketoACIDOSIS and diabetic patients without diabetic ketoACIDOSIS. Methods: This quasi-experimental study was performed by case-control method. The case group consisted of 30 patients with diabetic ketoACIDOSIS who referred to aAliasghar children hospital which is subset of Iran university of medical science in Tehran city for one year. The control group consisted of 30 patients from diabetic patients without ketoACIDOSIS referred to this hospital. The most important inclusion criteria for case group in this study was: age under eighteen years, diabetic ketoACIDOSIS with polyuria and polydipsia, ph under 7. 3, fasting blood sugar above 250 milligram in deciliters, keton in urine and not taking treatment for example intravein therapy and insulin infussion before admission in Ali asghar hospital and inclusion criteria for control group was diabetic patients without symptom and sign of ketoACIDOSIS. At the time of admission, blood samples were taken from patients with diabetic ketoACIDOSIS and evaluated for blood gas indices including HCo3, PCo2, pH, determination of serum levels of thiamine, sodium, potassium, hemoglobin, blood sugar, lactate and HbA1C. Serum thiamine levels were also measured in patients without diabetic ketoACIDOSIS. For determine of serum levels of thiamine in two groups the samples of whole blood were froze in under 70 degrees centigrade and sent to the lab then measured serum levels of thiamine by HPLC method. After admission of patients, initial measures for the treatment of diabetic ketoACIDOSIS including hydration with isotonic fluids, insulin injection at a rate of 0. 1 Unit / Kg / hr were performed according to the same protocol for all patients in the case group. All demographic information of patients in both groups along with the results of laboratory tests and patients' consciousness status scores were recorded in information forms for each patient and analyzed and evaluated using SPSS software version 23. Results: According to the research finding in fifty percent of all patients of our study including diabetic ketoACIDOSIS patients and nondiabetic ketoACIDOSIS patients the serum level of thiamine was low. In this study 56. 7 percent. of patients with diabetic ketoACIDOSIS had thiamine deficiency at the time of admission and in patients without diabetic ketoACIDOSIS 43. 3 percent had thiamine deficiency In the present study, the serum level of thiamine at the time of admission in patients with diabetic ketoACIDOSIS was significantly lower than patients in the control group. HbA1c levels were also significantly higher in diabetic ketoACIDOSIS patients than in controls. Also, serum thiamine level in patients with diabetic ketoACIDOSIS had a statistically significant relationship with HbA1c level. It means that poor control blood sugar is accompany with lower serum level thiamine. The present study is the first study that investigate serum level of thiamine in type one diabetic children and relation of it with HbA1C. Also results of this research show that diabetic ketoACIDOSIS treatment worse the thiamine deficiency. There was no statistically significant relationship between serum thiamine level in diabetic ketoACIDOSIS patients with lactate level, ketoACIDOSIS severity, Hemoglobin, Blood sugar, sodium, potassium, and Blood urea nitrogen. The results showed that there was no statistically significant relationship between serum thiamine level in patients with diabetic ketoACIDOSIS and age, sex, severity of diabetic ketoACIDOSIS, history of diabetic ketoACIDOSIS and duration of symptoms. Conclusion: The results of the present study showed that the treatment of diabetic ketoACIDOSIS exacerbates thiamine deficiency in these patients. Although the relationship was not statistically significant, a decrease in thiamine levels was observed. Therefore, it is recommended to consider thiamine supplements in children with diabetic ketoACIDOSIS. On the other hand, the weaker the blood sugar controls, the lower the thiamine level. Therefore, it is recommended that due to the side effects of thiamine deficiency in diabetic patients, all of these patients be encouraged to eat a healthy diet include foods containing thiamine in the diet for example fish, poultry, rice, oat.