Grape is one of the most important horticultural products in the world and Iran and according to latest statistics of the world food organization (2012), Iran has ninth place of producing grapes in the world. Grape black rot disease with the agent of Guignardia bidwelli is the most important disease of the grape in these regions, that is contagious and causes a 80-percent loss. In this study, in order to identify the grape black rot fungi in the Golestan province, a sampling of the leaf, stem and bunch of grapes was done in spring and summer in 2015-16. In order to isolate the symptom, the samples were transferred to the laboratory and after washing and disinfection with a 2% hypochlorite solution; they were cultured on a water-agar diet and also by culture of herbal tissues under wet conditions. Preparation of pure fungal cultures was done by single sporulation and tipping on a diet of potato dextrose agar. The optimal temperature for growing and producing picnides is 25 ° C and for germination 30 ° C. By cultivating the symptoms, yellow mycelia began to grow and picnithiosis appear as black dots after one week. This fungus, in addition to peccidiods of cyanoforms, also forms percithiomes containing ascospores in rotten and mummified fruit. Identification of the symptom causing the fungus was done according to the appearance of the parasitic of the fungus and the presence of the fetal organs on the host, cone or cylindrical tufts, undamaged, transparent, ovoid, oval or often round with dimensions of 4-7 x 5- 12 micrometers and surrounded by mucilage pods, ascorporeal group with neck endings outside the host tissue, double-glazed occipital, crown-shaped and 8 ascospores with dimensions of 9-14 x 45-65 micrometers, transparent, oval or ellipsoidal ascospores and without strap dimensions of 6-7.5 x 12-17 micrometers and using authoritative writings. Based on the available data, G. bidwelli was identified as the cause of Grape black rot disease. This is the first report of this fungi for Iranian mycobiota.