With random block experimental design, the soil microbial biomass carbon, soil temperature, soil moisture, and litterfall input in secondary oak forest and Pinus taeda plantation were measured in successive two years at the Xiashu Experimental Forest of Nanjing Forestry University. The results showed that in the two stands, soil microbial biomass carbon had an obvious seasonal fluctuation, being lower in plant vigorous growth season but higher during plant dormancy. The microbial biomass carbon in 0-10 cm soil layer ranged from 267.8 mg x kg(-1) to 459.8 mg x kg(-1) in P. taeda plantation and from 278.6 mg x kg(-1) to 467.8 mg x kg(-1) in secondary oak forest. Soil microbial biomass carbon had a significant negative correlation with soil temperature, but no significant correlations with soil moisture and litterfall input. It was suggested that the seasonal fluctuation of soil microbial biomass carbon in test stands could be more related to the availability of soil carbon and other nutrients, competition of plant roots for soil nutrients, and plant growth rhythm.