In order to examine the short-term effects of simulated nitrogen (N) deposition on soil organic carbon (SOC) in Stipa krylovii steppe, an N addition experiment was set up in May 2011 on a typical steppe, Stipa krylovii community, in Inner Mongolia, China. The experiment included five simulated N (NO₃^-) deposition level treatments, namely N1 (2 g N m^-2 a^-1), N2 (5 g N m^-2 a^-1), N3 (10 g N m^-2 a^-1), N4 (25 g N m^-2 a^-1), and N5 (50 g N m^-2 a^-1), and a control (CK) treatment with no N addition, each with three replicates. At the end of the growing season in 2011, soils at 0-2 cm and 2-10 cm depth were sampled separately. The fractionation of SOC was determined, and the potential SOC mineralization was measured in an incubation experiment. The SOC fractionation results indicated that particulate organic carbon (POC) was more sensitive to N deposition than the mineral associated organic carbon (MOC). The POC in 0-2 cm and 2-10 cm soils was higher in the N1 and N2 treatments, but lower in the N3, N4, and N5 treatments than in the CK treatment. The POC was significantly higher in the N1 than the N4 and N5 treatments. The ratio of POC to SOC can reflect the stability of the SOC pool. In the 0-2 cm soil, the POC/SOC was significantly higher in the CK than the N4 and N5 treatments, and in the 2-10 cm soil it was significantly higher in the N5 treatment. During the 70 day incubation experiment, the potential SOC mineralization decreased rapidly with soil depth. In the 0-2 cm soils, the potential SOC mineralization decreased in the order: N2 > N1 > N4 > N3 > CK > N5, with significant differences between the N1, N2, N3 and N4 treatments on one hand and between the CK and N5 treatments on the other. In the 2-10 cm soils, the potential SOC mineralization decreased in the order: N2 > N1 > N3 > CK > N4 > N5, with significant differences between the N1, N2, and N3 treatments on one hand and between the CK, N4 and N5 treatments on the other. Thus, simulated N deposition caused an increase in SOC, in particular POC, at low N addition levels, whereas at high N input, POC, MOC and the accumulated SOC mineralization decreased. Net primary productivity (NPP) was also affected by N addition. The NPP and aboveground biomass level was significantly lower in the N5 treatment than in the CK and other treatments, the underground biomass at 0-10 cm was significantly higher in the N1 treatment than in the CK and other treatments, and the amount of litter was significantly higher in the N5 than in the CK treatment. The effects of N deposition on NPP, biomass and litter were the main factors that influenced the SOC fractionation and the potential SOC mineralization in the short term. However, results showed that after one growing season, there were no significant effects of simulated N deposition on the SOC of Stipa krylovii steppe.