Aiming to exhibit the soil fertility of larch plantation forests based on the soil microbial biomass, the soil microbial biomass C and N were observed in two soil layer (i.e., humic layer and mineral layer) of two larch (Larix olgensis) plantation forests with different ages (nine years old: young plantation forest; forty\|three years old: mature plantation forest) for one growing season in northeastern China. Meanwhile, the relationships between the seasonal dynamics of soil microbial biomass and the soil nutrient and soil water were also analyzed in this study. The results showed that microbial biomass C and N were higher in humic layer than those in mineral layer for both larch plantation forests. In humic layer, the microbial biomass C and N were higher in young plantation forest than those in mature plantation forest. In both spring and autumn, microbial biomass C and N in the same soil layer were significantly different between two plantation forests (P<0.01). In humic layer, the microbial biomass C remained constant in young plantation forest through the growing season, while it reached maximum in autumn in mature plantation forest. The microbial biomass N was the highest in summer in humic layer for both plantation forests. In mineral layer, the microbial biomass C and N were the highest in autumn for both plantation forests. Microbial biomass C and N had a significant positive correlation with soil organic carbon and total nitrogen, but exhibited no significant correlation with soil water in both larch plantation forests. In addition, the microbial biomass C and N were also influenced by species composition and quantities of shrubs，and seasonal fluctuation of temperature in the stands. The above results suggested that: (1) the seasonal dynamics of soil microbial biomass C and N in the test stands were closely related with soil nutrients, and (2) the soil nutrients in young plantation forest were better than those in mature plantation forest.