In this study soil microbial biomass nitrogen (SMBN) in 6 soil samples were initially labelled with the ¹⁵NO₃^- in the incubation experiment.Then the NaOH hydrolyzable method was adopted to measure the alkali hydrolyzable-N in the soils, and winter wheat was grown on the soils in a greenhouse to evaluate the availability of SMBN to plants.It found that the extractability ratios of the NaOH hydrolyzable-N to the SMBN were in the range of 147 to 283, with an average of 201;there was about 30.1%~61.6% (averaging 42.9%) of NaOH hydrolyzable-N in the soils derived from SMBN.The atom % of ¹⁵N in the plant shoots grown on the soils ranged from 0.749% to 1.162%, which were higher than the natural atom % of ¹⁵N in soil, indicating the release of ¹⁵N immobilized by SMBN in soils.Therefore, it is concluded that the chemical and biological availabilities of SMBN are high.