In this study, the seasonal variation of different types of particulates was investigated in a fixed roadside station in heavily trafficked urban area of Hong Kong. Aerosol samples for total suspended particles (TSP), PM 1 0 and PM 2 . 5 were collected from June 1998 to May 1999 at a roadside site. Meteorological conditions such as relative humidity (RH), rainfall and prevailing wind direction were found to affect the mass concentration of TSP, PM 1 0 and coarse particulates at roadside level. Large size particles had an apparent seasonal variation, with higher concentration level in winter and lower in summer. The dry continental winter monsoon and the wet oceanic summer monsoon are the dominating factors. On the other hand, annual variation of PM 2 . 5 is relatively insignificant, suggesting that they are mainly from local traffic emission. PM 1 0 accounted for 62% of the TSP, while PM 2 . 5 accounted for 46%. The annual PM 2 . 5 /PM 1 0 is high with PM 2 . 5 responsible for 74% of PM 1 0 . In our heavily trafficked roadside fixed site, TSP exceeded the annual average of the Hong Kong Air Quality Objective by a factor of 1.53 while PM 1 0 exceeded by 1.39. The annual average concentration of PM 2 . 5 exceeded the National Ambient Air Quality Standard (NAAQS) annual average of 15μg m - 3 by a factor of 3.8 and is a cause of concern. A total of the 24h average PM 2 . 5 exceeded NAAQS by 33%. According to our data reported, fine particulate pollution is serious in Hong Kong.