Soil nitrogen (N) transformation processes determine the capacity of soil N supply to plant. The soil N transformation rates are affected by temperature and substrate availability. However, we are still far from understanding how soil N availability and nitrous oxide (N₂O) flux respond to warming and straw returning. We conducted a field experiment in a Mollisol cropland in Northeast China. We measured soil N availability and N₂O flux in the 0 - 20 cm soil layer by using ion exchange resin membrane (IERM)in situand static chamber-gas chromatography. Our results show that neither warming nor straw returning has significant effect on soil N availability, but the sampling time has a significant effect on soil N availability. Straw returning significantly increases N₂O flux; in July, warming and straw returning has a significant interaction on N₂O flux. Under the un-warming condition, straw returning significantly increases N₂O flux by 109%; but under the warming condition, straw returning reduces N₂O flux by 2%. Warming, straw returning, and sampling time has significant interactive effects on N₂O flux. Overall, this study helps to evaluate the effect of straw returning on soil N transformation under global warming in cropland ecosystems, which is essential tofuture research on N cycling in Mollisols.