A Mach-Zehnder modulator based on photonic crystal and nanowire waveguide is proposed. The modulator consists of silicon-based photonic crystal slab waveguides, nanowire waveguides, and two photonic crystal multimode interference couplers (MMI). Designed tapered structures are working at the junction of the photonic crystal and the nanowire waveguide to reduce the loss caused by the mode mismatch. The simulation analysis by time-domain finite difference method (3D-FDTD) shows that the insertion loss and extinction ratio of the modulator with a compact size of only 46μm × 8μm × 0.22μm at the operating wavelength of 1550 nm are 0.3dB,15.1dB,respectively. The operation with a wavelength range of 20 nm are obtained at a center wavelength of 1551 nm.The 3dB bandwidth can reach 68 GHz,which allows high speed optical communication systems.