A silicon-on-insulator 2×2 Mach-Zehnder thermo-optical switch is developed based on strongly guided paired multi-mode interference couplers. The multimode-interference couplers were etched deeply for improving coupler characteristicssuch as self-imaging quality, uniformity and fabrication tolerance. The proposed switch achieves good performances,including a low insertion loss of –11.0dB, a fiber-waveguide coupling loss of –4.3dB and a fast response speed measuredto be 3.5 and 8.8 μs for raise and fall switching time, respectively.