A practical application of leaky waveguide mechanism in quaternary long wavelength semiconductor lasers is given in this paper. It theoretically describes the possibility for application of the leaky waveguide mechanism in the quaternary semiconductor materials and the semiconductor long-wavelength laser devices with the related structure are designed. The devices are then grown by LPE technique. It shows that the theory and experiments agree well with each other. Therefore, the leaky waveguide mechanism is used in the semiconductor long-wavelength lasers theoretically and experimentally. This is a successful effort for the further applications of the leaky waveguide mechanism in other kinds of semiconductor lasers.