VC carbides are the common strengthening phases in the hot-work die steels containing V element, and gas nitriding is a popular thermo-chemical surface treatment, which has been using to improve thermal/mechanical fatigue and wear resistance of steels. In this study, the effects of VC precipitation on the hardness, thickness and wear resistance of the nitrided layer in AISI H13 hot-work die steel were systematically investigated. The results showed that a part of carbon atoms in the fine VC carbides were replaced by the nitrogen atoms during nitriding at 550 degrees C and some nitrogen were released to the matrix when the sample was cooled down to room temperature, so as to promote the hardness of the diffusion layer. Besides, VC precipitation promoted the diffusion of nitrogen by decreasing the C and V concentrations in the matrix, which decreased the diffusion activation energy of nitrogen in the matrix and made the diffusion layer thicker. Moreover, the thicker and harder nitride layer improved the wear resistance of the sample at a load of 100 N.