Abstract: The microstructure of the nitriding layer of H13 die steel after gas-nitriding under quenching and different tempering treatment conditions was studied by OM, SEM and XRD. The hardness of the cross section of nitricling layer was measured by microhardness tester, and the influence of tempering treatment on the layer was analyzed. The results show that the nitriding specimen after quenching and twice tempering at 350 ℃ had a compact diffusion layer, and the hardness of samples changed gradually. After quenching and twice tempering at 600 ℃, the nitriding specimen had thicker nitriding layer, but tiny cracks generated beneath the compound layer and the hardness of samples showed large scale change. The nitriding specimen after quenching had not obtain the nitriding layer with good comprehensive properties.