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An optical electric field sensor using a Pockels crystal which has the advantage of directly measuring the electric field in a space with limited disturbance of its distribution has been studied. We present an optical waveguide technique which has been developed to miniaturize the sensor. In a prototype sensor a light beam is propagated along a z-direction waveguide in a y-cut LiNbO3crystal. The width of the waveguide is 7 µ m and its length is 4.39 mm. The sensor is not based on an interferometer but on a single waveguide. The sensor has a simple and optimum configuration, and its output has good linearity to applied electric fields. Its frequency and temperature characteristics are presented. A new and smaller sensor with a waveguide 1 mm in length has been developed. It has almost the same configuration as the prototype sensor except for a smaller size. Results of measurement of ac and step electric fields using this sensor are presented.