High beam quality and ultrashort pulse laser is important for industrial applications like laser material processing. The Yb: YAG lasers have been studied extensively because Yb: YAG has several advantages for high power ultrashort oscillation due to low thermal loading and broad emission spectrum for the diode-laser pump scheme. We have developed a high average power ultrashort Yb: YAG master oscillator-power amplifier system based on the Yb: YAG regenerative amplifier. Average output power of 10.3 W has been achieved with 6.2 ps duration and 100 kHz repetition rate.