In this paper we theoretically investigate the nonlinear behaviour of a rectangular liquid crystal waveguide. An optical beam of a C-Band laser, fiber coupled to the waveguide, modulates the refractive index of the liquid crystal core. Light propagation is obtained when the effective refractive index of the core exceed the cladding refractive index one. The cut-off condition can be determined by the initial alignment of the liquid crystal molecules and by the laser beam itself that excites optical nonlinearity due to optically induced reorientational effect. Low power optical control could be obtained with an optimization of the molecular alignment.