Third-order nonlinear optical response of heat-treated magnesium ferrite (MgFe2O4) synthesized by simple combustion was studied by Z-scan experiment under continuous wave laser excitation (532 nm, 50 mW). Powder XRD patterns confirm the single-phase formation of spinel structure MgFe2O4 with cubic symmetry. Estimation of crystallite size from XRD data shows a decrease in size as annealing temperature increases. In the FTIR spectra, two absorption bands around 400 and 600 cm−1 correspond to the octahedral and tetrahedral sites of Fe–O and Mg–O, respectively. FESEM analysis pictures the presence of micro-grained particle with porous structure. Single-beam Z-scan experiment shows that the samples possess saturable absorption and self-defocusing nature. The third-order nonlinear optical properties calculated from the experimental data were consistent with the values predicted by theoretical fit. Estimated third-order nonlinear optical susceptibility, χ(3), was found to be increasing with increase in annealing temperature. The samples exhibit limiting behavior beyond 14.5 mW, and the corresponding output value gets clamped at 1.79, 1.95, and 2.54 mW. Magnesium ferrite treated at 900 °C with higher nonlinear optical susceptibility can be used as a potential candidate for low power limiting applications. © 2015, Springer Science+Business Media New York.