From transition-rate-equations and transmission-equations the formulas of threshold power are deduced for an endpumped laser, in which the probability of cross-relaxation cannot be ignored and the nonradiative transition-probability of the excited state is the same order a.s the radiative transition-probability of upper level of the lazing transition. The lasing performance of Tm:YVO4 at λ=1.9 and 1.5μm is discussed by dint of these equations of threshold power.