The design of an amorphous silicon-based Mach-Zehnder electro-optic modulator including two guiding p-i-n structures integrated inside a two-dimensional (2-D) photonic crystal (PhC) working at 1.55 μm, is reported. Electrically induced free carrier dispersion effect in this photonic material with a very cost-effective technology, is investigated for modulation. Our numerical analysis, performed by a time-domain (FDTD)-based software, proves that the voltage-length product can be remarkably reduced by taking advantage of both the strong PhC confinement and the wide refractive index tunability of amorphous silicon.