Abstract: In order to study the gain and thermal aberration properties of cryogenic Yb:YAG amplifier, a liquid nitrogen cooled Yb:YAG laser amplifier was built. Small signal gain for different pumping intensity and thermal aberration at room temperature and cryogenic temperature were measured. The results show that, with less pumping energy higher gain could be obtained at cryogenic temperature than at room temperature. At room temperature with pumping current 200 A and pumping duration 1200 s, the small signal gain is 1.59; at cryogenic temperature with pumping current 200 A and pumping duration 400 s, the small signal gain is 1.82. The optical to optical efficiency has increased observably. The effect of amplified spontaneous emission (ASE) is more evident at cryogenic temperature. Using short duration pumping can reduce the effect of ASE. The effect of thermal management was increased significantly at cryogenic temperature, and the laser amplifier can run at a higher average power.