In this paper, we present numerical calculation to study the effect of temperature on lifetime of laser levels, optical gain and output power in an unintentionally doped AlGaN/GaN quantum cascade laser. In the calculations, we have included the effects of internal polarization fields, and different scattering mechanisms using self-consistent numerical calculation of Schrödinger, Poisson and laser rate equations. The analysis of our results clearly indicates that the III- nitride based quantum well cascade lasers are one of the best coherent sources for the generation of the terahertz waves. Also, the threshold current density, the optical gain and output power were analyzed at different temperatures. A comparison of our results with the experimental data has shown good agreement.