Silicon-on-insulator (SOI) technology offers tremendous potential for integration of optoelectronic functions on a silicon wafer. In this letter, a 1 × 1 multimode interference (MMI) Mach-Zender interferometer (MZI) thermo-optic modulator fabricated by wet-etching method is demonstrated. The modulator has an extinction ratio of -11.0 dB, extra loss of -4.9 dB and power consumption of 420 mW. The response time is less than 30 μs.