Abstract: The effects of the various process parameters (tempering temperature before nitriding, nitriding temperature, nitriding time and resolution ratio of NH3) on the depth and micro-hardness of H13 steel nitride layer were analyzed and compared comprehensively through the orthogonal experimental design. The optimized nitriding process was given, and was compared with the original process before optimization. Results show that nitriding temperature was the most obvious factor for the hardness of nitride layer, and with the increase of nitriding temperature, the surface hardness increased first, then decreased. Resolution ratio of NH3 was the most obvious factor for the depth of nitride layer, and with the increase of resolution ratio of NH3, the depth of nitrided layer decreased, then increased. The optimized nitriding process was obtained as following: the tempering temperature before nitriding was 560 ℃, the nitriding temperature was 535 ℃, the nitriding time was 15 h, and the resolution ratio of NH3 was 50%. Compared with original nitriding process, the nitride layer of H13 steel prepared by optimization process had a higher hardness and a larger depth.