Introduction: The Complement system, consisting of more than 20 soluble proteins, has a crucial role in innate immunity and inflammation and eliminates pathogens and viral infections. Therefore, we investigated the titers of C3, C4, and total IgG in the sera of non-severe and severe coronavirus disease 2019 (COVID-19) patients. Methods: For this purpose, peripheral blood samples were collected from 30 non-severe and 30 severe COVID-19 patients and 30 healthy individuals with similar age and sex as the control group. The serum levels of total IgG, C3, and C4 were analyzed. Also, white blood cells, platelets (PLTs), and lymphocytes were counted by an auto-analyzer. Results: White blood cell count showed no difference between COVID-19 patients and the control group. The results showed a significant decrease in lymphocyte and PLT counts in COVID-19 patients compared to the control. Complement proteins, including C3 and C4, were increased in non-severe COVID-19 patients (C3,P = 0. 017 and C4,P = 0. 012) compared to other groups. Total IgG showed a notable decrease in severe COVID-19 patients. Conclusion: The decrease in C3 and C4 Complement factors in severe COVID-19 patients may be due to further consumption secondary to the formation of immune complexes. By clarifying the role of Complement proteins of C3 and C4 in different stages of the disease, our results can be helpful in designing therapeutic and diagnostic measures for the disease.