The output properties of passively Q-switched Nd:YAG/Cr4+:YAG ceramic laser was studied theoretically and experimentally. The repetition rate, the average output power and the single pulse energy were measured for different output coupling transmissions. The experimental results were compared with the theoretical calculation results. It can be shown that the repetition rate of output Q-switched laser decreases with the increasing of the output coupling transmission, whereas the average output power and the single pulse energy firstly increase to maximum and then decrease as the output coupling transmission increases. However, the optimal output coupling transmissions for the maximum average output power and the maximum single-pulse energy are different.