In order to investigate the effect of N on the microstructure and room temperature mechanical properties of new-type high silicon martensitic heat-resistant steels, three steels containing the same total content of C and N but different N contents have been designed and prepared according to the thermo-calc calculation. The thermodynamic calculation and experiments indicate that the replacing of C by N changes the kind and volume fraction of precipitates of the high Si martensitic steel significantly. Along with the N content increasing, the precipitates in the samples after 750 °C tempering change from (Cr₂₃C₆ + VN + TaC) to (Cr₂₃C₆ + VN + TaC + TaN) and finally to (Cr₂₃C₆ + VN + Cr₂N) according to both experimental results and thermodynamic calculations. The room temperature mechanical tests show that the strength of the steel decreases as the N content increases. However, the Charpy impact toughness increases with N content increasing. According to the calculation and SEM observation, it is inferred that the decrease of amount and size of precipitates accounts for the changes of the mechanical properties.