The study on the light amplification mechanism of iron-doped lithium niobate crystals has been done systematically.The experimental results indicate that the l ight amplification depends on the c-axis direction of the crystal. We propose a new view for the light amplification mechanism of the two-wave mixing in iron-do ped lithium niobate crystals: The diffusion mechanism cannot be neglected, and t he light amplification mechanism in doped lithium niobate crystals is caused by both the diffusion and the transient energy transfer. The decrease of the amplif ied light intensity after the steady-state is due to the decreases of the trans ient energy transfer with time and the energy dissipation caused by the beam fan ning.