There is provided an optical accelerometer including a first substrate, a second substrate, a spacer and a processing unit. The first substrate includes a frame, a movable member and at least one elastic member. Periodic slots are formed on the movable member along at least one direction to be served as a diffraction grating. The elastic member is connected between the frame and the movable member. The second substrate includes at least one sensing unit configured to detect a diffraction pattern formed by the diffraction grating. The spacer is disposed between the first substrate and the second substrate to define a predetermined height. The processing unit is coupled to the sensing unit and configured to calculate a 3D acceleration according to the diffraction pattern.