A novel grating light modulator for projection display is introduced. It consists of an upper moveable grating, a bottom mirror, and four supporting posts between them. The moveable grating and the bottom mirror compose a phase grating whose phase difference is controlled by the actuating voltage. When the phase difference is 2k\pi, the grating light modulator will switch the incident light to zero-order diffraction; when the phase difference is (2k-1)\pi, the grating light modulator will diffract light to first-order diffraction. A 16\times16 modulator array is fabricated by the surface micromachining technology. The device works well when it is actuated by a voltage with 1-kHz frequency and 10-V amplitude. The fabricated grating light modulator can show blackness and brightness when controlled by the voltage. This modulator has potential applications in projection display system.