In order to examine the effect of ply thickness on the crack initiation and propagation in the 90° layer in [0°/90°_n/0°] laminates, we conducted numerical simulations using two-dimensional mesoscopic numerical models. We found that the stress increase in the thin layer with 40 µm thickness was restricted in the vicinity of the adjacent layers, leading to restriction of crack penetration through the 90° layer. In addition, we confirmed the effect of stiffness of adjacent layers. In the case where the 90° layer was sandwiched between 45° layers, which had lower stiffness than the 0° layers, crack propagation in the 90° layer was faster than that observed with the 0° adjacent layers. Thus, the crack propagation behavior in the 90° layer was significantly influenced by the change in the stiffness caused by the orientation angle of the adjacent layers.