The order of magnitude of the cross relaxation propability between active ions and sensitive ions in Tb,Tm∶YVO 4 crystal and the cross relaxation among active ions in Tm∶YVO 4 are close with that of the non radiating transition probability and the radiant transition probability of higher level to lower level. The formula to estimate the laser threshold in the crystal including two sort codoped ions must take into account the cross relaxation among the ions. The estimated formula of the laser threshold including the probability of the cross relaxation among the ions is deduced from the transition rate equations and transmission equations of lasing and pumping light for end-pumped laser. The lasing performance of Tm∶YVO 4 and Tb,Tm∶YVO 4 at 1.5 μm and 1.9 μm wavelengths is discussed with these threshold equations, the action of Tb 3+ ions is also expounded. The results show that doping Tb 3+ ions in Tm∶YVO 4 increases intensity of lasing light and minifies the laser threshold at 1.9 μm wavelength.