Abstract: In this paper, a nonvolatile optical switch based on the phase-change material germanium antimony tellurium (GST) is proposed, and the nonvolatility of GST reduces the static power consumption in the optical switch. The device consists of a normal silicon waveguide and a silicon-GST hybrid waveguide, and the device dimensions are determined by phase matching of the effective refractive index and the mode field distribution in the hybrid waveguide. The mode field distribution in the hybrid waveguide is simulated and two supermodes are obtained. A strong coupling in the hybrid waveguide is found when GST is in the amorphous state. The simulation results show that the optimum operating wavelength of the device is 1573 nm, and the extinction ratios are as high as 22.5 dB and 30.8 dB in the amorphous and crystalline states, respectively. The insertion losses are as low as 0.17 dB and 1.86 dB in these two states, respectively.