We report total-reflection active-mirror laser experiments by using a cryogenic Yb:YAG composite ceramic. The composite ceramic has no high reflection coating on the bottom surface, and is cooled with liquid nitrogen directly. We obtained 273 W output power with optical efficiency of 65% and slope efficiency of 72% against the absorbed pump power. The laser power and optical efficiency will be improved more when the pump power increases further. To investigate thermal effects of the laser material in more detail, we have measured the thermal lens focal length and the temperature of Yb:YAG. We observed thermal lensing effect of f similar to 2000 mm, and the maximum temperature of 150 K for 400 mu m-thick Yb:YAG sample. We have also studied the theoretical analysis of thermal distribution in the composite ceramic.