Abstract: The paper aims to illustrate the changes of labile organic nitrogen in arable mollisols under different organic matter levels after long-term fertilization and discuss the evolvement mechanism of farmland mollisols fertility responding to fertilization. Using the long- term experiment on mollisols productivity, the authors investigated the effects of 8-year continuous application of chemical fertilizers on labile organic nitrogen in arable mollisols under different organic matter levels. Farmland mollisols with five different soil organic matter (SOM) levels, 11% (Beian), 6% (Nenjiang), 5% (Hailun), 3% (Dehui) and 1.7% (Lishu), were taken as the study objects. Compared with none-fertilization treatment, the long-term application of chemical fertilizers increased the contents of soil organic nitrogen, including microbial biomass nitrogen, particulate organic nitrogen, and light fraction organic nitrogen (except dissolved organic nitrogen). Soil organic nitrogen, microbial biomass nitrogen, particulate organic nitrogen, and light fraction organic nitrogen all decreased with the decline of SOM. Particulate organic nitrogen contributed the most to labile organic nitrogen in all mollisols. Fertilization had no effect for the correlation between organic nitrogen and active organic nitrogen in the studied mollisols.