Using cw Ti:sapphire laser as pumping source, the passively Q switched Yb:YAG laser with Cr4+:YAG as a saturable absorber has been demonstrated at room temperature. An average output power as much as 55 mW at 1 03 μm with a pulse width (FWHM) as short as 0.35 μs was abtained. The initial transmission of Cr:YAG has influence on the pulse duration (FWHM) and the repetition rate of Yb:YAG passively Q switched laser. The experiment shows that Yb:YAG crystal will be a most promising passively Q switched laser crystal for compact, efficient solid state lasers.