The effect of nitrogen on the mechanical properties of 20MnSi steel at high temperature was investigated. Mechanical tensile test was conducted using a Gleeble-1500D. Microstructure and fractograms were analyzed using SEM and TEM. The results indicate that nitrogen in concentration of 0.4 ~ 0.6% improves the grain boundaries and grain refinement, and thus the strength and section shrinkage and plasticity of the steel. Further increase in concentration of nitrogen has little effect on section shrinkage.