Abstract: Being the most active part of soil organic matter, microbial biomass can be used to characterize soil fertility, and plays an important role in maintaining balance of the soil ecosystem. The dafumigation-extraction method (FE) was used to determine soil microbial biomass carbon content (C_mic) and nitrogen content (N_mic) in three types of urban forest soils in Shanghai for analysis of their spatio-temporal and seasonal variations. Results show that the soils varied sharply in C_mic and N_mic with the type of forest. C_mic in the soil of the Cinnamomum camphora forest, Metasequoia glyptostroboides forest and C. camphora-dominated mixed forest varied in the range of 46.57-908.77, 64.58-480.54 and 57.38-853.18 mg·kg^-1, respectively, while N_mic in the range of 10.61-103.64, 20.14-88.39 and 16.67-148.00 mg·kg^-1, respectively. Both C_mic and N_mic declined with soil depth in all the soil profiles. However, soil microbial biomass did not vary much seasonally in the >15-20 cm soil layer, but quite significantly in the 0-5, >5-10 and >10-15 cm soil layers, in all the types of forest soil, and often peaked in autumn. Correlation analysis indicates that the content of soil microbial biomass was significantly and positively related to the content of soil organic carbon and total nitrogen(P<0.01), but negatively to soil bulk density(P<0.01). Moreover, N_mic was greatly influenced by soil water content and soil temperature. As a result of the joint effect of vegetation and environmental factors, the three types of forest soils differed in content of soil organic carbon and total nitrogen and hence the spatio-temporal distribution of soil microbial biomass.