A residual stress is one of the key parameters for determining the mechanical properties of SiC/SiC composites. The differential swelling between composite constituents is anticipated under irradiation, which likely modify the unirradiated residual stress. To clarify the magnitude of residual stress modification by irradiation and the effects on mechanical properties of SiC/SiC composites, the swelling in each component were evaluated. The volume expansion of the matrix and fibers in NITE SiC/SiC were measured at 1.9 and 1.4~1.7 %, respectively, following 5.1 MeV Si 2+ ion irradiation at 280 o C, 3dpa-SiC. The calculation showed the decrease in the axial residual stress in matrix region, where the increasing in the proportional limit stress was implicated.