Abstract: The photocathode sensitivity is one of the most important and fundamental parameters of image intensifiers, which determines the performance of the photo-electronic imaging system under low-light-level conditions. Based on the physical model of semiconductor optoelectronic emission and Plank theory of black-body radiation, the effect of 5 sub-processes on photocathode quantum efficiency is summarized. Several expressions are given to calculate the sensitivity limit under the assumption of 100% sub-quantum efficiencies in the 5 sub-processes. The ultimate sensitivity of 6569μA/lm is obtained for an extended blue GaAs photocathode in the waveband of (0.41～0.93)μm.