The nucleation of diamond on the amorphous SiO2mirror surface has been achieved by means of adding an ac signal to the negative dc bias in the microwave plasma chemical vapor deposition. It is found in experiment that the nucleation of diamond happens only after the frequency of the ac signal exceeds a threshold. The results also show that the diamond nucleation density depends not only on the ac frequency but also on the magnitudes of the ac signal and the dc bias. ©1995 American Institute of Physics.