Abstract:14 different soils under tobacco-rice rotation in southern Hunan Province were collected to study the effects of rice-straw-returning on the contents and components of organic nitrogen fractions in soils based on Keeney and Bremner’s classification of organic nitrogen. The results showed rice-straw-returning only increased the content of unknown hydrolysable nitrogen rather than the contents of ammonia nitrogen, amino acid nitrogen and unhydrolysable nitrogen; Rice-straw-returning enhanced the correlations between ammonia nitrogen, unknown hydrolysable nitrogen, unhydrolysable nitrogen and organic matter, total nitrogen, alkali-hydrolyzable nitrogen, while weakened the correlations between amino acid nitrogen, amino sugar nitrogen and organic matter, total nitrogen, alkali-hydrolyzable nitrogen.