We studied Isotropic-Nematic (I-N) phase transition by nonlinear dielectric measurement. The third-order nonlinear permittivity epsilon(3) was obtained front the third harmonic component of the electric displacement under AC electric field. The obtained epsilon(3) shows the critical behavior in I phase near I-N transition point with the critical component of one. This result agrees well with the critical behavior predicted by mean - field theory, but that of the relaxation time deviates from the theoretical prediction.