Using laser diode (LD) as pumping source, CW Yb:YAG laser output was obtained at room temperature. The pumping threshold of Yb:YAG is as low as 2.2 W, the maximum output power of 5.2 W was attained with the pumping power of 18 W, and slope efficiency is as high as 33%. At the same time there is no saturation in laser experiments, so the high power output of Yb. YAG laser can be obtained with high power diode laser pumping source. This research will be beneficial to the development of high power Yb:YAG lasers.