It is generally known that the crack initiation strain is increased with thinner 90° layer.However the mechanism has not been well clarified in the case of very thin thickness as thin as 40μm.A systematic detailed experimental observation was made on the damage growth behavior in 90° layer of cross-ply CFRP laminates.As a result of observation, the damage mode and the damage growth behavior are different depending on the ply thickness of 90 layer.It was clarified that crack extension was restrained and crack extension speed became slower as becomes thinner.Moreover, it was shown that crack opening displacement becomes smaller as layer thickness is thinner and the energy release rate at the crack tip becomes smaller based on FEM calculations.