A non-paraxial equation for nonlinear optical propagation in the scalar approximation is derived. As a first-order correction, it is shown that the nonparaxiality can be expressed with a fourth-order spatial dispersion and fifth-order nonlinearity. By using variational approach to the modified nonlinear equation the propagation properties of Gaussian beams are obtained. It is found that the beam propagates stably with a focusing-defocusing cycle, and the beam with a higher power has a smaller cycle. It is also shown that the self-focusing process can be controlled by changing the chirp parameters. The positive (negative) chirp reduces (increases) the initial self-focusing length and both chirps increase the focusing-defocusing cycle.