This survey spanned two years, between 2004 and 2006. Blood samples were collected from 431 stray dogs distributed along North of Iran (Golistan, Mazandaran and Gilan provinces), the coastal areas of the Caspian Sea. The Knott's modified test was used for diagnosis of Dirofilaria immitis and other filariae. Meanwhile, the periodicity of microfilaria in peripheral blood circulation was calculated and the imaging diagnosis techniques included radiographic, echocardiography and electrocardiography of four dogs that had positive results with Knott's test, were done. Diagnostic parasitology results indicated that 16.01% of stray dogs were microfilaremic. Two different microfilariae were diagnosed: D. immitis in 13.69%, Dipetalonema reconditum in 1.86% and in 0.46% both of them. There was no statistically significant between infection to filariae with gender (P = 0.868) and age (P = 0.083) of dogs. Also study on the periodicity of the presence of microfilaria in peripheral circulation showed that the highest rate of those was at 1 am (13316±57 microfilaria) and the lowest rate at 12 pm (6681±23 microfilaria). Radiographic study showed distinctive signs with varied degrees of severity included: Tortuous and enlargement of main and lobar pulmonary artery, pulmonary parenchymal lesions (bronchial, Interstitial, Vascular and Alveolar pattern) and Right side heart enlargement that confirmed in electrocardiography. Also in echocardiographic images observed short parallel sided images with the appearance of equal signs that indicated the presence of the heartworm. These results showed that to obtain a reliable diagnosis of heartworm infection, imaging tests can support parasitological exams.