Many phenomena associated with nonlinear optics are produced by the light-matter interaction in liquid crystals. Nematic liquid crystals possess the properties of the birefringence and that refractive indexes of nematic liquid crystal vary with temperature. As a light beam propagates in liquid crystals, the light beam experiences changes of refractive indexes because the optical field reorientates molecules or the optical intensity changes the temperature of liquid crystal. Then, some interesting phenomena of optical nonlinearity produce in liquid crystals. This study investigates mainly the nonlinear behaviors in nematic liquid crystals. By etching ITO glasses to control distribution of electric field, we discuss applications in photo-electric field. These works are described as follows: First, a low voltage was applied to a planar nematic liquid crystal cell; the director field can be reoriented using a low intensity. Then, the self-focusing effect produces due to a variation of refractive indexes. The light beam in nematic liquid crystal forms a spatial soliton by producing the effect of the self-focusing to balance the diffraction. Additionally, we study the interaction between solitons. One soliton creates a potential well of refractive index, anther one will be attracted in the potential well. As the separated distance between two solitons and the pumping angle are appropriate, two solitons propagate in the form of spiral. Second, we study the behavior of light in a periodic refractive index medium. The director field of the nematic liquid crystal (NLC) is reorientated in a grating¡Vlike indium-tin-oxide electrode cell by applying a controllable-voltage. The variation of refractive index with voltage varied 0v to 10v was observed by a conoscopic method. Numerical simulations have reproduced the main features of the gradient distribution of refractive index in the waveguide. Several phenomena of a polarized laser beam that propagated in the waveguide with different incident angles and positions have observed by a CCD camera, including solitons, undulate beam, the total internal reflection and beam coupling. Third, at the temperature close to nematic-isotropic phase transition temperature, the variation of refractive index in the liquid crystal becomes obvious to the change of temperature. And, a laser beam can easily reorientate molecules. We changed beam intensity in liquid crystal cell, different nonlinear phenomena were observed. Besides, A combined microscopic and conoscopic technique was used in experiments as a convenient way to analyze the optical nonlinearity that is associated with the molecular configuration of nematic liquid crystal.