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In this paper, we present the design, fabrication, and characterization of a new micromechanical grating optical modulator. By applying a dc driving voltage to the electrostatic comb-drive actuator, the grating beams can be reconfigured. Experimental tests reveal that at a voltage of 45 V, the grating period can be altered from the initial 10 to 20 mum, and correspondingly, the plusmnfirst-order diffraction angle is changed from 3.86deg to 1.94deg for a normally incident laser beam (wavelength, 632.8 nm). While the measured modulation of optical intensity can be as large as 30.6 dB for +first order and -19.1 dB for +second order. The adoption of silicon-on-glass process for fabricating this grating optical modulator is simple, high yield, and with a very smooth optical surface. The modulator has a first-level resonant frequency of 21.539 kHz, enabling a fast-enough manipulation. When operated in a digital mode, the proposed grating optical modulator can be used as an optical switch and an analog mode for a variable optical attenuator. Both show potential applications for the future optical network.