To identify the Alternaria spp. pathogenic to tomato and potato, surveys of tomato and potato fields were made during 2009- 2010 in West Azerbaijan province. In total about 261 isolates belonging to Alternaria genus were isolated and purified. Based on morphological studies 6 species, namely: A. alternata, A.tenuissima, A. interrupta, A. arborescense, A. brassicina and A. tomatophila from tomato and 3 species viz. A. mimicola, A. humuli and A. brassicina from potato were identified. Four species of: A. alternata, A. tenuissima, A. arborescense and A. tomatophila had been already reported on tomato crop previously. In this study it was found that 3 species of A. mimicola, A. humuli and A. brassicina were new to the mycoflora of Iran, These three species, were isolated from potato along with two other species of A. interrupta and A. brassicina , isolated from tomato , were reported for the first time accross the world. Pathogenecity studies of the isolates of identified species were done on the potato cultivar Agria and the tomato cultivar 282 BSS, Their pathogenecity was being through Koch’s principles. All the studied species were pathogenic on potato and tomato, although the pathogenic isolates belonging to various species differed from each other. The most abundant belonged to species A. tenuissima and A. arborescense while A. arborescense, and A. tomatophila species were recorded as the most pathogenic.