Communities of periphytic diatoms in Yangsan stream were investigated from September 1989 to August 1990. A total of 122 taxa of periphytic diatoms belonging to 39 genera, 104 species, 16 varieties and 2 forms were identified. The number of species was more in fall and winter than in spring and summer and the important dominant species were Nitzschia palea, Cocconeis placentula, and Navicula cryptocephala. The mean of annual species diversity was 3.01 and the saprobity was oligosaprobity. Evenness showed decreasing tendency from upper reaches to lower reaches of Yangsan stream. There were no differences in similarity indices among sampled stations. Correlation coefficient was relatively high between dominancy and diversity, dominancy and evenness, and diversity and evenness.