The third-order nonlinear optical susceptibility, (3) , of lanthanide (lanthanum, praseodymium, neodymium, and samarium) borate glasses has been measured by the third harmonic generation method. The structure of the present glass system has been studied by infrared and Raman spectroscopic methods. The network structures of the present Ln 2 O 3 -B 2 O 3 glasses have been confirmed to be basically similar to each other. Praseodymium, neodymium, and samarium borate glasses exhibit (3) values that are larger than lanthanum borate glasses, because of the optical resonance effect, in accordance with the f-f transition. Especially, the (3) value for 30Pr 2 O 3 ⅐70B 2 O 3 glass is 1.8 × 10 −12 esu, which is a factor of ∼60 larger than that of SiO 2 glass. This striking enhancement of (3) is mainly attributed to the large transition moment to the first excitation state.