Solid nitriding of the C422 (22Cr12NiMoWV) steel was carried out to investigate microstructure of the nitriding layer and notch sensitivity of the nitride steel, which provided a technical support for evaluating service reliability of the C422 steam turbine valve stem. Composition, phase and mechanical property tests, as well as cross-sectional and fracture surface observations were employed to clarify microstructure of the nitriding layers under different nitriding temperatures and times, and also effect of nitriding on mechanical properties of the C422 steel samples with/without notch. Accordingly, notch sensitivity of the nitrided steel was estimated. The results show that surface nitriding layer of the C422 steel is mainly composed of Fe3N, Fe4N, Cr23C6, CrN, VN and WN. After nitriding at 550 ℃ for 6 h, the nitriding layer of 80 μm in thickness is dense, and has high hardness. As increasing the nitriding temperature and time, the thickness of the nitriding layer is not apparently changed, but the coarse microstructure and surficial microcracks are detectable. The C422 steel samples with/without nitriding both have a notch sensitivity ratio less than 1, indicating that the steel is sensitive to notch, and nitriding increases notch sensitivity. After nitriding, the yielding-to-tensile ratio/the plasticity of the C422 steel sample increases/decreases, meanwhile cleavage of the nitriding layer suggests an apparent brittle fracture characteristic. The optimal solid nitriding parameters of the C422 steel are the nitriding temperature of 550 ℃ and the holding time of 6 h. The nitriding layer is dense and high in hardness. Nitriding has no apparent effect on the notch sensitivity of the C422 steel, but results in embrittlement of the steel.