In this study, we focused on the geometrical change of cross-ply laminates that are widely used for the composite structures. The behavior of time-dependent out-of-plane deformation under high temperature and humidity was examined. The CFRP plates were exposed at 80°C, 90% RH and their change in geometry was measured at arbitrary elapsed time. The initial shape of the specimens was twisted saddle shape, and the shape gradually became flat with time. Up to 500 μm of out-of-plane deformation arose during moisture absorption for CFRP symmetric plate with the size of 280×280×2.4 mm. The displacement in the thickness direction was the most along ±45° directions in which reinforcing fiber was not aligned. Finite element analysis (FEA) was performed to simulate the out-of-plane deformation of cross-ply laminates. The diffusivities D and coefficient of moisture expansion β, which were obtained other experiments in authors' previous work (Arao et al.: Adv. Compos. Mater., 17 (2008), 359-372), were used. The ply angle misalignment was considered as factors of generating out-of-plane deformation. For the analytical results, it was confirmed that the model the surface layer of which was rotated 1° deformed approximately 700 μm, and the plate geometry became twisted saddle shape by moisture absorption.