The effect of gas nitriding and subsequent ageing on the microstructure and properties of 2Cr13 steel was investigated using X-ray diffractometer, scanning electron microscope, transmission electron microscope, microhardness tester, wear tester, bend test and immersion corrosion test. It was found that with the increase of nitriding temperature and time, both of the thickness of nitrided layers and surface porous layer increased. The diffusion layers of the sample nitrided at 500 ℃ mainly consisted of martensite, a large amount of dislocation and dislocation cell structure, as well as a large amount of CrN nano-precipitates. The CrN nano-precipitates dispersedly distributed in the interior of grains and at the boundaries of dislocation cells, and the CrN nano-precipitates at the boundaries of dislocation cell were denser and larger than those in the grain interior. Further, a large amount of ε-Fe2-3 N or (Cr,Fe)2N were precipitated in the grain boundaries of lath martensite. When the nitriding temperature increased up to 550 ℃, the martensite recovered itself, and the CrN precipitates lined up to form CrN lamellas. The microstructure of the diffusion layers was also transformed into the lamellar pearlite that consists of the α-Fe matrix and CrN phase. The surface hardness of nitrided samples first increased and then declined with the increase of nitriding temperature, where the peak hardness (1 274 HV0.5) reached at 500 ℃. On the other hand, the hardness of diffusion layers formed between 450 ℃ and 500 ℃ was greater than 1 300 HV0.1, and obviously decreased with the nitriding temperature. The wear resistance and corrosion resistance of 2Cr13 steel were improved after nitriding at 500 ℃ for 5 h, but the bend toughness decreased. No matter water cooling or air cooling or subsequent tempering of 420 ℃ for 2 h was applied, the variance of the microstructure at the diffusion layers of 2Cr13 steel nitrided at 500 ℃ was limited, and their hardness was also not changed obviously and was still greater than 1 000 HV0.1.