Third-order nonlinear optical susceptibility, χ⁽³⁾, of the sol-gel derived α-Fe₂O₃ thin film has been evaluated by the third-harmonic generation (THG) method. χ⁽³⁾ of α-Al₂O₃ single crystal (1120) with the same corundum structure as α-Fe₂O₃ has also been determined. The χ⁽³⁾ of the α-Fe₂O₃ film was 5.8×10^-11esu, which is comparable with those of organic materials with high nonlinearity. The large χ⁽³⁾ value of the α-Fe₂O₃ film may be ascribed to the high refractive index of 3.00 and to the three-photon resonance as well. The χ⁽³⁾ of α-Al₂O₃ single crystal was 2.9×10^-14esu, which is three orders of magnitude lower than that of α-Fe₂O₃ film and almost the same as that of fused silica. The small χ⁽³⁾ value of α-Al₂O₃ single crystal was attributed to the small refractive index and wide optical band gap.