This paper Introduces the application of CWEOP (continuous wave electro-optic probing) technique in the measurements of electric field distributions in the samples of LiNbO3 directional coupling waveguide modulator. Two configurations of measurement——the backside incidence and the end incidence have been taken in the experiments. The curves of electric field distribution under the electrodes, and in the region between two eleotrods and in the in terrupt region of electrodes have been obtained. The experimental results have been discussed.