The X-ray diffraction method was used to measure the residual stress in the compound and diffusion layers of gas-soft nitrided carbon steel (JIS S45C) and low-alloy steel (JIS SCM435). The X-ray elastic constant of the compound layer made of ε phase iron nitride was determined experimentally. The residual stress was tension on the surface and gradually changed to compression in the compound layer. The residual stress was compression in the diffusion layer near the interface with the compound layer. The compression zone extended about 0.7mm for S45C and 0.35mm for SCM435 from the surface. In this compression zone, an increase in Vickers hardness was observed. The tensile fracture strength of the compound layer measured by the X-ray method was about 600MPa for S45C, and 700MPa for SCM435.