The design of an amorphous silicon based Mach-Zehnder electro-optic modulator integrated into a two-dimensional (2D) photonic crystal (PhC) has been designed for wavelengths of~ 1.55 [micro] m. The numerical analysis has been performed by using a finite-difference time domain (FDTD) based simulation tool. The design of the PhC-based MZ Modulator relies on the exploitation of the refractive index tunability of the hydrogenated amorphous silicon (a-Si: H) for the fabrication of PhC waveguides requiring minimum critical technological processes.