Efficient passively Q-switched microchip laser based on Yb:YAG/Cr 4+ :YAG composite crystal has been demonstrated under high brightness single-emitter laser-diode pumping. Maximum average output power of 1.5W was obtained when the absorbed pump power was 3.65W, the corresponding optical-to-optical efficiency was over 41%. The slope efficiency was 52.3%. The effect of the cavity length on the performance of Yb:YAG/Cr 4+ :YAG composite crystal passively Q-switched microchip lasers was investigated. Laser pulses at 1030nm with pulse width of 466ps and peak power of 91kW were achieved with cavity length of 1.7mm, while laser pulses with pulse width of 665ps and peak power of 79kW were obtained with cavity length of 3.7mm.