A high repetition rate diode pumped Nd:YAG laser, passively Q -switched with a Cr4+: YAG plate, has been analysed. lt shows that the output energy and the pulse width depend on the pump energy, the initial transmitance of the Cr4+: YAG and the reflectivity of the output mirror. Based on the results of the calculation, an experiment has been performed and a 12 ns Q -switched laser pulse output of 13 mJ under pulse repetition rate of 100 Hz is obtained.