We report here the design and fabrication of an electromechanical tunable grating on silicon-on-insulator (SOI) wafer. The tunable grating consists of a submicron electrostatic comb actuator and an expandable freestanding grating. Rigorous coupled-wave analysis (RCWA) method is utilized to analyze the optical responses of freestanding grating with different periods and filling factors. Obvious shift of the resonant peaks is obtained by changing the grating period and the grating filling factor. The electromechanical tunable grating is realized on the silicon device layer by a combination of electron beam (EB) lithography, deep reactive ion etching (DRIE) and wet etching. Scanning electron microscope (SEM) micrographs indicate that the grating is well fabricated. Via applying biased voltage, the force generated by the electrostatic comb actuator can modulate periods and filling factors of the freestanding grating. The electromechanical tunable grating with simple fabrication process shows bright prospects for optical telecoms and miniaturized spectrometers.