A guided-wave electro-optic Mach-Zehnder interferometric modulator was fabricated on X-cut $LiNbO_3$. The channel waveguides were formed by proton diffusion with self-aligned $SiO_2$-cladding. In order to observe the electro-optic effect of proton-diffused $LiNbO_3$ waveguide, a electro-optic cutoff modulator with self-aligned electrode was fabricated and tested at 0.6328㎛. The modulation depth of the cutoff modulator was over 90% at applied voltage of 25V. Almost linear voltage-intensity relationship has been observed. A Mach-Zehnder interferometric modulator with arm lengths of 7mm has been fabricated. Its modulation depth with $V_\pi$ of only 3.5V was 85＼% and the 3dB bandwidth was 1.6KHz. For high speed operation, the electrode dimension should be reduced to have smaller R, L and C.
