This paper studied the dynamics of microbial biomass N in a black soil long-term fertilized with farmyard manure (M2 and M4) and chemical fertilizer (NPK) during crop growth season. The results showed that the soil microbial biomass N was 25.52-239.12 mg x kg(-1) in M4, 10.40-94.31 mg x kg(-1) in M2, 6.27-87.04 mg x kg(-1) in NPK, and 9.15-69.81 mg x kg(-1) in CK. In same treatment, the highest value was 7-14 times higher than the lowest value. Treatments NPK and M2 had the highest value of microbial biomass N at reproductive growth stage, while M4 and CK had it at vegetative growth stage. The significant difference of soil microbial biomass N caused by different treatments was not changed with plant growth stages. The dynamics of soil microbial biomass N had a significant correlation some soil biological, physical and chemical properties. A significant positive correlation was found between soil microbial biomass N and soil biological, physical and chemical properties, plant N, P and K contents, soil moisture, and grain crude protein.