Various liquid crystals can be infiltrated into interconnected nanosize voids in a three-dimensional periodic array of SiO2 spheres, synthetic opals, as photonic crystals. As an example of tunable photonic crystals, it has been demonstrated that the optical stop band in transmission spectrum of the liquid crystal infiltrated opal shifts with changing temperature, which can be interpreted in terms of temperature dependence of the refractive index and confirmed by the theoretical calculation of the band structure. This phenomenon can be utilized as a method to evaluate the refractive index. Tunability of optical properties of photonic crystal is also demonstrated in infiltrated replicas of opals. Effects of electric field application on properties of the infiltrated opals are also clarified.