Field trials were conducted in a manural loessial soil with medium fertility, located in Yangling, Shaanxi, to study the changes of the mineralisable N and the relationship between its contents and biomass N in soil, the soil in different layers being sampled and the mineralisable and biomass N determined at regular interval times. The results show that the distribution of the mineralisable N varied with layer as well as with plant growth stage: being higher in cultivated layer than in deep layer, and lower and stable in early stage while 1 time higher in late stage. Application of N fertilizer had no obvious influence on the mineralisable N whereas application of organic fertilizer increased its contents in late stage of the plant growth. The effects of kinds of plants and plantation systems(i.e., with plantation or with no plantation) on mineralisable N were much less than sampling times. The mineralisable N released during plant growth period was only 12% of the biomass N, and there was no correlation between them. It seemed that the biomass N could hardly reflect the N-supplying capacities of a soil.