Monthly variation of mutagenicity by airborne particulate were studied according to particle size of the particulate. Airborne particulates were collected in Shinchon of Seoul which is commocial and traffic area in 1986. And those were separately collected into two parts such as fine particle (less than $2.5{mu}m$ aerodynamic diameter) and coarse particle (greater than $2.5{mu}m$). Extractable organic matters(EOM) were extracted and mutagenicity of the EOM was tested in Salmonella thyphimurium TA 98 by Ames method. While the concentration of coarse particle did not show the seasonal variation, that of fine particle showed great seasonal variation. The contents and mutagenicity of EOM in fine particles were higher than those of coarse particles. So fine particles were expected to contribute to the 90% of mutagenicity in atmosphere by suspended particulates. The content of EOM and mutagenicity by suspended particulates in atmosphere were highest in January all the year around and also higher as much as 6 and 30 times than in July, respectively.