Cytospora canker caused by the fungus Cytospora spp. is one of the most common diseases of walnut trees in many parts of Iran. To identify the species of Cytospora on the walnut trees, during 2008-2009, more than 200 samples of infected twigs and branches with symptoms of Cytospora canker were collected from the major walnut cultivation areas in Iran. Based on morphological characteristics of asexual fruiting bodies on the bark (including presence or absence of conceptacle, shape, size and number of ostiol, shape and color of disk, arrangement of chambers, color of tendrils, size of conidia, conidiophores and conidiogenous cells and branching type of conidiophores) four species, namely Cytospora cincta, C.chrysosperma, C.leucostoma and C.schulzeri were identified. C.chrysosperma consisting of about 80% of collected isolates was determined as prevalent species. Moreover, the walnut tree was known as a new host species for C. schulzeri.