Seasonal variation of microbial biomass affected by hydrological condition was studied in a temperate forested wetland, dominated by Alnus japonica, in suburban area near Seoul, Korea. One upland site (US) and two wetland sites, named drained site (DS) and poorly drained site (PDS), were selected based on the hydrological condition. From June 23, 2008 to March 20, 2009, the 0-15cm mineral soils were sampled for every 45 days. Microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) were measured using chloroform-fumigation extraction method. Mean MBC and MBN (mg/kg) of US, DS and PDS were 166.0 and 15.7, 432 and 26.7 and 898.1 and 48.3, respectively. Mean microbial CN ratio of US, DS and PDS was 9.6, 17.0 and 22.8, respectively. There were significant differences in MBC, MBN and microbial CN ratio among sites. MBC of DS and PDS was higher in the spring season and the autumn than in the summer, however there was no significant seasonal variation of MBC in US. Seasonal variation of MBN, higher in the spring and the autumn and lower in the summer, was found only in US. Microbial CN ratio was not significantly different among the season. Microbial biomass and its seasonality were highly affected by the hydrological condition.