Abstract: Based on the FresnelKirchhoff diffraction theory, this paper calculated the intensity distribution of the farfield diffraction patterns when a divergent or a convergent gaussian beam transmitted through a thin nonlinear medium, and analyzed the effect of the different Kerr mediums and the different gaussian beams on the patterns formation. The simulation results show that, when a divergent gaussian beam transmits through a defocusing nonlinear medium or a convergent one through a focusing nonlinear medium the pattern in the far field is a series of thick, larger diffraction rings with a central dark spot, while when a divergent gaussian beam transmits through a focusing nonlinear medium or a convergent one through a defocusing nonlinear medium the pattern in the far field is a series o