Hybrid oospores are formed as a result of the cross of the A1 and A2 two mating types of oomycetes Ph. infestans. The aim of this study was the observation of hybrid oospores in the infected and collected leaves from potato fields and confirmation of the presence of A2 mating type in Zarrin Gol region of Aliabad-e Katul of Golestan province. In May 2017, during the several visits on the potato fields of the western regions of Golestan province, specially in Zarrin Gol region of Aliabad-e Katul, samples with blight symptoms, particularly leaves with two areas of joined together necrosis were collected and transferred to the laboratory. Samples were incubated for 24 hours at 18°C in the dark conditions in wet chambers. The small pieces of each leaflet (5-10 mm2) were prepared from the lesion and it’s margin. To discolour and clearly microscopic observation of oospores, samples after being placed in small beakers, immersed in 96% ethanol, were placed for 3-5 minutes in a water bath and left for 30 minutes. The samples were immersed in three times their volume in 10% sodium hypochlorite for one hour. Optical microscope observations on the base of examination of 10 light microscopic fields (0.2 mm2) at 10x objective lense, showed oospores in the range of 51-250 with oospore production index at medium (OPI=2). All of the observed oospores had antheridium. Therefore, there was no parthenogenetic oospores. Oospores had 25-35 µm in diameter and thick walls around them. Therefore, oospores were hybrid. To ensure the presence of A2 mating type, isolation and purification of the pathogen were conducted on rye B agar culture media with tetracycline antibiotics and benomyl fungicide at 500 ppm. Ph. infestans isolates were paired with standard tester strains A1 Ph. infestans on rye B agar with tomato juice. The mating type of each isolate by observing hybrid oospore was determined A2 and conversely, A1. The results showed that the ratio of the A2 to A1 mating types ratio was approximately 1 : 6 in benefit of A1. The presence of the A2 mating type provide the possibility of sexual reproduction and genetic diversity of Ph. infestans field populations in Zarrin Gol region of Aliabad-e Katul of Golestan province. This increases the probability of emergence virulent population and resistant to fungicides.