Mach-Zehnder interferometer (MZI) 2×2 thermo-optic switches, based on parabolically taped MMI couplers, have been designed in this paper. The theory of the parabolically tapered MMI coupler is studied to estimate the optimal length, and then the effective index method (EIM) and the beam propagation method (BMP) are utilized to simulate these devices to find out the optimal length. The values of waveguide parameters are set as: the refractive index difference is chosen to be 0.75%, the width of the MMI field is 30 micron, the distance between the MZI arms is 100 micron and the separation between the access (input/output) waveguides is 250 micron. The simulated results show that the performances of the tapered coupler is better, whose the second width is equal to 25 micron. MMI coupler is also simulated by BPM. Although the tapered coupler can not offer performances as well as those of other MMI coupler, it can offer compact devices. The switches, based on conventional MMI couplers and tapered couplers, are simulated at the same of access waveguide separation and MMI width. Simulated results show that although the performances of this new structure are not as good as those of the switches based on conventional MMI couplers, but can reduce the length of the device effectively. This compact characteristic is important for integrated planar optical circuit.