Electroluminescence (EL) under alternating-current (ac) operation is first reported for n+-polysilicon/SiO2/p-Si MOS capacitors with 50 nm Si-implanted SiO2. Visible EL can be observed with the naked eye in the dark. The ac operation by pulse-wave distinctly enhances the EL intensity and its lifetime. The pulse frequency affects the EL spectrum and thus the EL color. A model of EL mechanism is proposed for the Si-implanted MOS EL device, which has a possibility of visible light emitting device.