Abstract: Incubating trials in lab and field were carried to study the effect of long-term returning rice straw on the characteristics and processing of soil organic N mineralization in tobacco field. Lab trials proved that long-term rice straw returning had a positive effect on the soil organic N mineralization with improved potential and rate. The mineralization potential of the 6-years returning soil was 92.11 kg/hm2, and was 23.78% greater than that in the non-straw returning. Compared with control, the rice straw returning could advance the soil N mineralization. The net mineralization in tobacco growth period increased by 17.67% and 15.30% by the rice straw returned 5-years and 1-year respectively. In the tobacco field soil incorporated rice straw, the amount of soil inorganic N was enhanced and effected the soil N mineralization positively during the middle and late growth period of tobacco. The soil microbial biomass N in straw incorporating soil was 20.18% greater than that in the non-straw incorporating at beginning of transplantation, and was 40.71% greater at ending of harvest stage. The soil microbial biomass N in rice straw returning soil alternate during the first year and did not be stable at a new level.