This paper reported the design of the optical switch using GaN/sapphire for telecommunication wavelength at 1.55 μm. The design of optical switch based on Mach-Zehnder Interferometer which is consists of two directional couplers connected by two rectangular arms. Design optimization was conducted by finite difference beam propagation method (FD-BPM) using OptiBPM software. Optimization was conducted by a varying waveguide parameter such as waveguide width, waveguide thickness, width gap, and coupling gap. From the numerical experiment, the best design obtained of the optical switch was 6380 μm long and 15 μm wide, with the electrode length, was of 4500 μm. The optical switch could operate as an optical switch at Vπ = 34V with and a power imbalance of 8.73 dB, an insertion loss of 1.20 dB, an extinction ratio of 8.73 dB.