In this study, we investigated analytically and experimentally time-dependent deformation of CFRP sandwich panels with polymer foam core ROHACELL due to temperature change and moisture absorption and desorption. Tensile tests for the ROHACELLs were carried out before and after the moisture absorption using digital image correlation method, and we obtained stress–strain relationship in each condition. Surface deformations of sandwich panels were calculated by the finite element analyses considering the elasto-plastic constitutive equation of the ROHACELLs based on the tensile test results. Experimentally measured deformation in the sandwich panels after the moisture absorption was approximately 100μm. This was much larger than the analytical result. The deformations remained in after re-dry condition. On the basis of this study, we suggest that the main reason for the deformation in the sandwich panels could be irreversible shrinkage of the ROHACELLs caused by absorbing moisture during exposed in high temperature and high humidity environment.