Anisotropic diffraction patterns were observed on far-field screen induced by a weak (0.16 W/cm2) linearly polarized collimated beam passing through a homeotropic C60-doped nematic liquid crystal (5CB) cell. Based on orientational photorefractive effect, two-wave coupling experiment was carried out to induce liquid crystal molecule re-orientation. The form of space-charge field with the Gaussian distribution induced by the Gaussian beam incident on the sample is investigated. The outline of far-field diffraction pattern is concentric ring, and shows symmetrical gaps in the direction perpendicular to the direction of polarization of light. In addition, diffraction pattern changes when the polarization of the incident light is changed, effective nonlinear index coefficient is n20.3 cm2/W. The results showed that our theoretical analyzsis was consistent with the observed experimental phenomena.