We report the fabrication and device applications of the nonlinear photonic crystals (PhCs) which combines PhC functions and highly nonlinear optical organic materials. We reveal a direct relationship between the observed nonlinear optical responses and the corresponding photonic band dispersion nature such as a slow light in the nonlinear PhCs, and we show experimental evidence that the performance of nonlinear optical applications such as the optical switch, wavelength conversion and two-photon absorption devices are dramatically improved in this structure. And moreover, we report on the design and experimental demonstration of a Mach-Zehnder electro-optic (EO) modulator based on the silicon PhC waveguides combined with our novel EO polymer exhibiting large electro-optic properties. We show the slow light enhancement of the EO efficiency in the device, and reveal a direct relationship between the observed slow light properties and the corresponding EO modulation characteristics.