A Mach-Zehnder Interferometer based optical modulator composed of photonic crystals as phase shifters is presented. The switching process relies on the phase modulation along the arms of interferometer resulted from the photonic band shift of the photonic crystal regions. A small bulk index change leads to a large effective index difference between two arms of the interferometer and a small foot-print device can operate as a modulator. Presented MZI-based optical modulator is shown to have tunable bandwidth up to 15.7nm. This interesting approach provides possibility of broadband, low-voltage, low-loss, high-speed and tunable optical integrated switches, modulators and multiplexers.