The third order nonlinear optical susceptibilities χ⁽³⁾ of Sb₂O₃-B₂O₃ binary glasses have been measured by the third harmonic generation (THG) method. The IR and Raman spectroscopic study reconfirmed that the structure of Sb₂O₃ glass is similar not to senarmontite but to valentinite which consists of double-chains of SbO₃ units. The formation of BO₄ units is much smaller for Sb₂O₃-B₂O₃ glasses than Bi₂O₃-B₂O₃ glasses at all compositions, which is ascribed to the covalent character of Sb-O bonds. It is also considered that Sb₂O₃ is present as SbO₃ trigonal units and participates in the glass network formation through the Sb-O-B bonds. The χ⁽³⁾ value increased monotonically with increasing Sb₂O₃ content up to 8.4×10^-13 esu for Sb₂O₃ glass, which is about 30 times larger than SiO₂ glass. Modified Lines'linear relationship between χ⁽³⁾ and (n_ω²+2)³·(n_ω²-1)·E_d/E₀² was adopted to Sb₂O₃-B₂O₃ glasses.