In this study, a new method for measuring the nonlinear refractive index of materials using the moir e deflectometry is reported. We describe the experimental details and present theoretical analysis for obtaining the magnitude and the sign of nonlinear refractive index of materials, n 2 , using the moir e fringe patterns. Optical-field induced refractive index changes in materials leads to well-known self-focusing/defocusing effects (self-lensing effect). In presence of a sample of nonlinear refractive material, the convergence or divergence of a collimated incident laser beam is monitored by the moir e deflectometry system. The results show that the moir e fringes rotate about the beam center and the fringes sizes change too because of the induced self-lensing effects in the sample. The nonlinear refractive index of materials and its sign is obtained by analyzing the moir e fringes patterns. The theoretical aspects of the method are presented and the relationship between the moir e fringes rotation and the nonlinear refractive index is derived. Our method is applied for measuring the nonlinear refractive index of Fe 2 O 3 nanoparticles in toluene solution under 5-mW He-Ne laser (k ¼ 632:8 nm) illumination. The value of n 2 ¼ À4:9 Â 10 À7 cm 2 W À1 is obtained for Fe 2 O 3 nanoparticles in toluene solution and it is in excellent agreement with the reported value in literature.