We report on the designing an optical modulator using photonic crystal (PhC) Mach-Zehnder interferometer (MZI) structure. Typical modulators use MZI structure, where the length of the device stretches up to several millimeters because the electro-optical effect of silicon is very small. We find that the group index of the PhC waveguide can be changed by changing the refractive index of the PhC air holes by filling in the air holes with electro-optic polymer materials. The change of the refractive index of the polymer in the hole by 0.005 induces the change of the group index of the PhC waveguide by up to 0.2. We find that the group index increases, when the wavelength and the radius of the electro-optic polymer hole increases and when the refractive index of electro-optic polymer hole decreases. We find that the group index increases when the length increases and that the group velocity and propagation loss depend on the length of the photonic crystal waveguide due to the finite length of the photonic crystal waveguide. Using all of these findings, we could obtain higher electro-optical effect, which reduces the length of the device into micron scale. When we apply the voltage, we can change the phase difference by π due to the electro-optical effect of the polymer. Our calculation shows the extinction ratio between 35 to 45 dB, dependence of length of the device.