Diamonds are wide-gap semiconductors possessing excellent physical and chemical properties; thus, they are regarded as very appropriate materials for optoelectronic devices. Based on the Kerr effect, we introduce a simple and feasible method for measuring the third-order nonlinear optical susceptibility of synthetic diamonds. In the experiments, synthetic type I diamond samples and transverse electro-optic modulation systems are utilized. As for the laser with the wavelength of 650 nm, the third-order susceptibility and Kerr coefficient of the diamond samples are obtained at χ⁽³⁾₁₂₁₂=2:17×10^-23 m²=V² and S₄₄= 1.93×10^-23 m²=V², respectively.