The material of the specimen was austenitic stainless steel, SUS316L. The residual stresses in the specimen was introduced by a water j et peening (WJP). The change in the residual stress with thermal aging at 773K was measured by an X-ray stress measurement. The WJP res idual stresses were an equi-biaxial stress state, and the compressive residual stress did not decrease against the thermal aging. To i nvestigate dependence of the residual stress on a lattice plane, the WJP residual stresses were measured using hard synchrotron X-rays.