Infection to Dirofilaria immitis in carnivorous especially dog is worldwide and was reported in broad areas of world also in different regions of Iran included: West Azarbayejan, Ardebil, Mashhad, Shiraz, Tehran and Khozestan. In present study, 110 dogs in different areas of Iran were trapped and after recording of the necessary information included: age, sex, breed and housing, were sampled. After isolation of their sera, they were examined with WITNESS HW test. For determining of the periodicity of the microfilaria in peripheral circulation, blood samples were prepared from a dog with 4 years old for 3 days (72 hours) any hour and these samples were examined with modified knott method. Then number of microfilaria in each ml of the blood samples were calculated and regarding to average of microfilaria in each hour was determined periodicity of the presence of microfilaria in circulation. Using by this method, prevalence of the infection to Dirofilariosis in Iran was 14.55%. Statistical analysis showed relationship between infection and housing significantly and these relationships between infection and age, sex and breed were not significant. Study on periodicity of the microfilaria in peripheral circulation showed that average of the number of microfilaria in each ml of the blood was maximum on 1 am (13316 microfilaria) and minimum on 12 am (6681 microfilaria). Therefore presence of the microfilaria in peripheral circulation in infected animal has type of the periodicity (nightly periodicity). In ELISA method, secreting antigens of adult worm of Dirofilaria immitis were estimated and diagnosis of infection has not related to presence or absence of microfilaria in blood circulation. So in case with occult infection or amicrofilariaemia that test with modified knott is negative, can be use from ELISA method (WITNESS HW test). Regarding to report of the infection in different regions and also infection of human to this parasite in Iran should be regard this helminth and necessary activities should be done for control of the infection.