An optical phase modulator for integrated optics was proposed by monolithic integration of deformable silicon photonic wires and MEMS actuators. Via applying voltage, the silicon wire is deformed and the wire length is extended, resulting in change of optical phase. A phase shift of 0.18 π is achieved at a bias of 35 V for a 150 μm wire according to numerical simulation. By cascading many of the MEMS-actuated wires, a π-phase shift can be realized at very low operation voltage. Based on this idea, a MEMS-actuated Mach-Zehnder interferometer was demonstrated.