To investigate the epilithic diatom community and water quality of the Osan stream, water samples were collected from the eight stations from April to September 2003. Sampling was two times before and after heavy rain. Total 52 diatom were identified and divided into 12 saproxenosus taxa, 6 saprophilous taxa and 34 indifferent taxa, respectively. The DAIpo values higher after heavy rain than before that. According to tolerance degree to the organic water pollution, all sampling stations ranged from $alpha$-oligosaprobic to $alpha$-mesosaprobic. Thus, the result indicates that the water quality of Osan stream is gradually improved by heavy rain.