A linear electrooptic modulator using crystalline quartz is proposed and experimentally investigated, and its optical bias is produced in the crystal itself. Required optical bias can be produced from the natural birefringence and optical activity of a crystal if the dimensions of the crystal and its dielectric axis azimuth to light propagating direction are properly designed. For crystals exhibiting both electrooptic Pockels effect and electrogyration effect, such as crystalline quartz, it is better to simultaneously make use of the two effects to increase its modulating sensitivity. The experiment on electrooptic intensity modulation using crystalline quartz is performed for the modulating voltage from 27 V to 4.5 kV, and the linear correlation coefficient between output signal and modulating voltage is no less than 0.9999.