The absorption and fluorescence spectra of Tm: YAG crystals doped with Cr3+ and Er3+ sensitization ions have been studied. The average lifetime of 2E, 4I13/2, 3F4 levels of Cr3+, Er3+, Tm3+ ions, energy transfer efficiencies and rates have been calculated. Cr3+→Tm3+ transfer efficiency is 80%, transfer rates is 2. 1 ×103s-1. The fluorescence spectrum of (Er, Tm): YAG crystal shows that Er3+→Tm3+ energy transfer is efficient one. It was discussed that the sensitization mechanisms are difference between Cr3+ to Tm3+ and Er3+ to Tm3+.