BACKGROUND: Cryptosporidium is an intracellular protozoan parasite that known as the major cause of diarrhea in human and animals. OBJECTIVES: The level of infection to Cryptosporidiumin diarrheic calves of livestock around Tehran and the role of risk factors on its emergence were studied. METHODS: 200 fecal samples from diarrheic calves of livestock’s around Tehran city were collected by cluster sampling method and were studied by modified Ziehl-Neelsen staining method. RESULTS: Existence of Cryptosporidium infection was confirmed in 18 calves (9%). Incidence rate of the infection did not have any significant relationship with sex of animals (p=0.928) and diarrhea history (p=0.640), but the incidence rate of the infection was more in suckling calves with age of less than 2 months with dry fertilizer bedding. CONCLUSIONS: According to the reports of Cryptosporidium infection in multiple studies from different regions of Iran, recognizing the risk factors on the infection incidence can have an important role in controlling the protozoan.