Abstract: In order to optimize the Al composition of the barrier in GaN-based quantum cascade laser, a self-consistent calculation to the schrodinger and poisson equations of one periods in QCL has been done. The band structure and the electron envelope function are obtained. The relation between dipole matrix mement and Al composition of the barrier is illuminated . The results show that the dipole matrix mement is largest with the optimized Al composition to be 0.15.