In this study, a non-distractive method of x-ray diffraction (XRD) was used to determine residual stress of rolling mill rolls made of graphite steel (GSH48). This method utilizes the variations of distance between crystal planes as strain. The determination of residual stress was performed samples in different depths before and after conducting ultrasonic peening technology. In UPT process, impacts were exerted on the workpiece ball tool, resulting in the improvement of some mechanical properties such as residual stress by creating work hardening and compression. After the simulation and manufacturing of ultrasonic vibratory tool and then the installation of that on lathe machine, UPT operations were conducted on the prepared samples. Measuring residual stress from surface to 0.5 mm depth was performed before and after the UPT process. After the numerical simulation of the UPT, the distribution of experimental residual stress and numerical simulation was compared that the results suggested the increase of compressive residual stress about 0.4 mm from the surface after the UPT process. The rise of compressive residual stress in the rolling mill rolls leads to the increase of their strength and fatigue life and as a result, their working efficiency is boosted. After the UPT process, the grain size of the surface was calculated from the model of the x-ray diffraction using Viliamson-Hall relation that grain size was obtained 60.2 nm. The refinement of surface structure arises because of displacement arrangement again due to vibration with high frequency and severe plastic deformation after the UPT process.