Optically injected semiconductor laser is applied to photonic microwave generation. The microwave is generated by the beat between the oscillation frequency of the slave laser and that of the external injection light. We find that the microwave frequency can be tuned from 61 GHz to 129 GHz by varying the injection optical power in experiment. The tunable ability in our scheme can also be achieved through adjustment of the frequency detuning. Numerical simulations indicate that the frequency of the optically generated microwave can be higher than 40 GHz by using the optical injection semiconductor laser technique.