The experimental results of a 2×2 integrated thermooptic Mach-Zehnder interferometer (MZI) switch based on multimode interference (MMI) couplers are reported. The insertion loss is 3.40 dB, polarization-dependent loss is 0.47 dB, extinction ratios at bar state and cross state are 32.01 dB and 16.42 dB respectively, response time is less than 3 ms, and power consumption is 658 mW. Regarding to asymmetry of extinction ratios at bar state and cross state, theoretical analysis is presented. Coupling ratio of the two multimode interference couplers deviates from 50:50, but the deviation is nearly the same, which is decided by good uniformity of semiconductor planar lightwave circuit (PLC) process. This property affects extinction ratios at bar state and cross state in different mechanisms. Adding metal heating electrode on waveguide affects its effective index and analysis based on experimental results is given. The increment of waveguide effective index is in the order of 2×10-4.