The optical gain of the wurtzite GaN/AlGaN quantum well lasers are theoretically studied on the basis of the first-principles calculation and the k . p theory. The parameters required in the analysis are derived from the first-principles electronic hand calculations by a full-potential linearized augmented plane wave method. The small spin-orbit splitting energy as well as very heavy hole masses plays an essential role in causing higher threshold current density than conventional zincblende lasers.