Abstract: in this paper, based on Hahn-Tsai nonlinear theory, energy release rate and the delamination-tip fields of delamination of a laminated composite were analyzed by the quasi-3D FEM. The results indicate that the effect of material nonlinearity along thickness direction on energy release rate G (specially on GII) are significant and related to the post-buckling model of the laminates. Around the delamination-tip interlamminar stresses are reduced and displacements are increased obviously.