In this paper, the electro optical properties of a KNbO3 single crystal was studied. A KNbO3 deflector with four cylindrical electrodes, which utilizes the coefficient γ33 of the EO tensor, was made. Results of the experiment indicated that the fundamental figure of merit of the KNbO3 deflector was about twice that of the LiNbO3 crystal with equal structural parameters, aperture and length.