Introduction: Intestinal protozoa of dogs have a relatively high diversity, and the prevalence of infection is often observed sub-clinically and sometimes clinically. Different species of Cryptosporidium are observed in the dog’s intestines. Due to the importance of pathogenicity of gastrointestinal protozoan parasites in dogs and also their zoonotic importance, this study was conducted on stray dogs in Tabriz. Methods: During this study, two hundred stool samples of stray dogs from different regions of Tabriz were prepared and examined microscopically for parasitology. The samples were tested by the Formalin-Ether concentration method, and also the following Ziehl-Neelsen modified staining method was used to observe Cryptosporidium protozoan. Results: Out of 200 fecal samples, 16 samples (8%) were infected with Cryptosporidium protozoa. Statistical analysis of the results also showed that there was a statistically significant difference in the rate of infection with protozoan parasites between dogs under one year old and dogs over one year old (P<0.05). Regarding the sex of dogs, there was a statistically significant difference in the prevalence of Cryptosporidium between male and female dogs (P<0.05). Sixteen dogs infected with Cryptosporidium parasite were also treated with tylosin for one week, and a decrease in infection with this parasite was observed in 9 treated dogs. Conclusion: In this study, the rate of Cryptosporidium infection in stray dogs decreased from 8% to 3.5% after tylosin treatment, and this drug may be useful in controlling this parasitic disease in dogs, but more extensive research should be carried out to eradicate this parasite in dogs.