Ovine theileriosis as a critical agent in small ruminant production, can cause lethal infections. Different species of Theileria have been reported in various parts of the world, and each species causes different diseases in the host. This is the first molecular study to investigate the prevalence of ovine theileriosis and identify the dominant Theileria species in northern Iran. A number of 220 small ruminants, including sheep and goats, were randomly sampled from 22 flocks. Peripheral blood smears were stained by the Giemsa staining method. As well as for species identification, all samples were examined by PCR. From 220 samples, 160 and 60 were sheep and goat, respectively. By the Giemsa staining method, Theileria parasite was observed in 20 (9%) samples. But by polymerase chain reaction (PCR) method, 30 (13.6%) samples were positive for Theileria species. Theileria lestoquardi was the most common species found in these animals. The high prevalence of theileriosis in small ruminants demonstrates the emergence of ovine theileriosis in Mazandaran and Golestan provinces in northern Iran.