A new electro-optic modulator is fabricated using langasite (LGS, La3Ga5SiO14) crystal, and the electro-optic effect of langasite single crystal is studied. The half-wave voltage and the corresponding electro-optic coefficient γ11 are determined by the extreme method as well as the double frequency method. It is shown that it is relatively easy, convenient to measure γ11 by the double frequency method compared with the extreme method. The specific optical activity of 21.1°～2.6°/mm in the range of 0.3～0.6328 μm of langasite is determined by using a new method. The electro-optic modulating experiment of langasite crystal has been completed with home-made high-voltage modulating power. The results show that langasite crystal having optical activity is a kind of new electro-optic crystal and suitable for fabricating electro-optic modulator.