Dirofilaria immitis, a common parasite of the cardiovascular system of the carnivores all over the world, is reported from dogs, cats and, humans in Iran. Knott method is the most common test for its diagnosis in many regions including Iran. In the present study, during one year period from 2002 to 2003, blood samples of 110 dogs were examined using modified Knott method and commercial antigen detect test kit (WITNESS CHW II kit). All dogs were subjected to necropsy to evaluate the specificity and sensitivity of both tests. In Knott method, 22 harbored dogs (20%) were microfilaraemic of which (of the first instance), 20 dogs (18.18%) D. immitis and 2 (1.82%) D. reconditum. Accordingly, by using commercial antigen detection test kit 16 dogs (14.54%) were found to be infected with D. immitis. At necropsy 14 out of 110 dogs (12.73%) harbored D. immitis in the right ventricle of the heart. Thus, it was concluded that 6 out of 20 positive dogs with D. immitis were microfilaraemic and these microfilaria belonged to other filarial except D. immitis. To compare the findings with golden test (necropsy), the sensitivity and specificity of Knott and antigen detection test were 85.71 and 91.66%, 92.85 and 96.87%, respectively. McNemar test showed that although sensitivity of the latter was higher, differences were not significant (p > 0.05).