Forward degenerate four-wave mixing (DFWM) processes are investigated with a femtosecond pulsed laser inlithium niobate crystal doubly-doped with magnesium and iron (LiNbO3:Fe, Mg). The pulse energy dependencereveals a pure third-order nonlinear response, and the third-order nonlinear susceptibility x(3) in the materialis evaluated to be 4.96 × 10-13 esu. The time-resolved DFWM process shows a response time of x(3) shorterthan l00 fs, which is due to the nonresonant electronic nonlinearities. Our results indicate that LiNbO3 crystalshave potentials for ultrafast real-time optical processing systems, which require a large and fast x(3) opticalnonlinearity.