Propagation of optical lattice soliton in nonlinear Kerr medium with harmonic modulation of refractive index is investigated analytically and numerically. The equations governing evolution of the soliton parameters and the conditions for soliton formation and stable propagation are obtained. It is shown that the beam is finally trapped in a guide-like channel and propagates stably when the launching angle is smaller than a critical value. The critical angle increases as the depth and period of modulation of refractive index increase, and increases as the beam width decreases. Furthermore, linear spatial chirp upsets the balance between diffraction and nonlinearity, thus affects the formation and stable propagation of the soliton, although it doesn't affect the central position of the beam. In order to maintain the soliton propagation one can offset the effect of the chirp by using proper beam power.