In this paper, a laser-diode-pumped Cr4+:YAG repetitively Q-switched Nd3+:YAG laser is studied theoretically and experimentally. The analytical expression of the pulse repetition rate including the effect of gain medium field splittings and finite lower level lifetime is derived. The pulse energies, pulse widths and repetition rates in different conditions are measured and the experimental results show rough agreement with the theoretical predictions.