Effects of moisture on the stress-strain response and damage progress of quasi-isotropic CF/ Epoxy laminates ([0°/45°/-45°/90°]s) are presented. Tensile tests are conducted for both dry and wet specimens to compare the stress-strain behavior and the damage processes between them. The transverse cracking and delamination are microscopically observed and quantitatively measured at various loads. Macroscopic residual hygrothermal stresses are calculated by using the classical lamination theory to discuss those effects on damage initiation. The Young's modulus and Poisson's ratio of specimens with edge delamination are predicted on the basis of a modified delamination model and compared with experimental results. From the above experiments and calculations, it is proved that nonlinearity observed in the stress-strain response is attributed to the large scale edge delamination throughout the length of the specimen. The moisture increases the critical stresses for transverse cracking and delamination by reducing the residual stresses, while it decreases the stress of the transverse extension of edge delamination along the width. The modified delamination model gives better agreement with experimental results than the conventional one.