Photonic crystals (PCs) are periodic structures made of materials with different refractive indices. The main feature of PCs is that electromagnetic waves are prohibited to propagate within a certain frequency range called photonic band gap (PBG). Materials containing PBG have many potential applications in optoelectronics and optical communication. For instance, a dielectric layered structure can be used to design as a Fabry-Perot interferometer, dielectric reflectors, and antireflection coating. In this thesis, using lithium niobate (LiNbO3) as an electro-optical defect layer in a quarter-wave photonic crystal, a theoretical analysis of the tunable resonance in a multilayer Fabry-Perot resonator (FPR) is given. With the fact that the refractive index of LiNbO3 is voltage-dependent, in the first part, we have investigated the tunable filtering properties in the visible and near infrared regions as a function of the applied voltage and of the angle of incidence for both TE and TM waves. In the second part, by varying the thickness of the defect layer, we found that a multichannel filter can be achieved. This multichannel resonant peaks in the transmittance appears to be the common feature for the defective PC filter containing the nonlinear defect like the LiNbO3 or the ferroelectric or even the metamaterial. The theoretical analysis in this thesis is based on the transfer matrix method. The format of thesis is as follows: The Chapter 1 is to give a brief review of PCs. The Chapter 2 describes the theoretical method used in our calculation. Some topics under study are arranged in Chapters 3 and 4, respectively. The conclusion is summarized in Chapter 5.