Firstly the tension experiment was undertaken to investigate the mechanic behaviors of the flange repaired composite panels in this paper. Emphases were played on the damage initiation and the failure strength for the repaired composite structure. Then numerical studies were conducted by using finite element method. The effect of geometric non-linearity on the stress-strain response was taken into account through quadrilateral, thick shell elements (S8R). A better agreement with experiment could be obtained when geometric non-linearity was considered. In addition, a user material subroutine (UMAT) was implemented into ABAQUS for studying damage initiation and its progression in the composite structure. Finally, all the predicted strain distributions, damage development and strength of the repaired laminate were compared with experiments. The predictions and experiments were found to be in good agreement.