An InGaAsP/InP 2×2 Mach-Zehnder interferometer (MZI) optical switch based on multimode interference (MMI) couplers was presented. The switch characteristics were simulated using a three-dimensional beam propagation model. The switch structure was designed to be single mode operation and insensitive to polarization. The light signal can be switched from cross port to bar port with an injection current of 43mA by carrier modulation on the principle of injection free carriers, bend-filling effects and bandgap shrinkage. The measured crosstalk is -19.2dB and -14.3dB at the on and off state, respectively. This optical switch can be easily integrated with other active devices and has potential application in future DWDM and OXC systems.