Temperature dependence of second harmonic generation (SHG) was examined theoretically by solving the coupling problem composed of heat conduction equation and complex wave amplitude equations. The latter equations include effect of laser absorption in the crystal. KTiOPO₄ (KTP) crystal was supposed as a nonlinear optical crystal, and its SHG characteristics were compared with KH₂PO₄ (KDP) crystal. Power density distribution in the crystal, time variation of the conversion efficiency and output-beam profile were analyzed quantitatively during repetitive irradiation of pulse laser in Gaussian mode. As a result, it was revealed that there are various mode in output-beam profile, such as ring mode, single mode with one or two rings in the surrounding, depending on incident power density or crystal length. These disorders of transverse mode are caused by the inverse conversion of SHG. It was also concluded that there is an optimum length of crystal to the irradiation conditions, and it can be estimated by the present study.