Optical properties of (101¯0)-oriented wurtzite (WZ) GaN/AlGaN quantum well (QW) lasers are investigated by the non-Markovian gain model with many-body effects. These results are compared with those of (0001)-oriented WZ GaN/AlGaN QW with spontaneous and piezoelectric polarization taken into account. The energy pattern of the (101¯0) orientation shows two-fold symmetry while the energy pattern of the (0001) orientation is completely isotropic. It is found that the many-body effects are more dominant in the (0001)-oriented QW compared to the (101¯0)-oriented QW. This is because the average hole effective mass of the (0001)-oriented QW is larger than that of the (101¯0)-oriented QW. Thus, the flatband model shows that the optical gain of the (101¯0)-oriented QW is smaller than that of the (0001)-oriented QW. However, with the inclusion of the internal fields, it is observed that the optical gain of the (0001)-oriented QW is significantly smaller than that of the (101¯0)-oriented QW. © 2000 American Institute of Physics.