Second‐order hyperpolarizabilities &bgr; of organic nonlinear optical materials were measured by a modified method of the electric‐field induced second‐harmonic generation. Since this method avoided interference between two second‐harmonic waves from a solution and cell‐window glass, &bgr; of a dilute solution was easily determined and at the same time, a third‐order hyperpolarizability &ggr; of a solvent was also evaluated. The obtained &bgr; values were constant in a wide range of the concentration.