In this study, a lab X-ray in-situ measurement technique was applied to quantify the relaxation behavior of compressive residual the mechanical surface modified layer of low-carbon austenitic stainless steel in high temperature environments. The surface modified specimens were prepared through shot peening with two different levels of processing degree. Then, two at elevated temperature, such as incremental temperature rise and isothermal holding, were carried out, respectively, specimens. The results showed that relaxation of the compressive residual stress was almost finished when the temperature value. In the high-temperature range only, compressive residual stress gradually decreased over the elapsed time. It noted that stress relaxation in high temperature environments were correlated highly with the reduction of full width at (FWHM) obtained by X-ray diffraction, regardless of degree of mechanical surface modification process. It was then relaxation behavior of compressive residual stress within mechanical surface modified layer under high temperature environment was thought to be mainly due to reduction of plastic strain induced by mechanical surface modification processes.