This study investigates the electro-optical phase and intensity modulation characteristics of the optically active and electro-optical crystal and defines its π-voltage. For electro-optic crystals with optical activity, the former concept of half-wave voltage cannot accurately describe the periodical characteristics of electro-optic polarization and intensity modulation. Thus, the π-voltage concept is introduced and defined as the modulation voltage required for the elliptical birefringence phase-delay variation of such a crystal to be equal to π. As for an optically active and electro-optical crystal intensity modulator inserted between two polarizers, the optical activity can provide optical bias for electro-optical intensity modulation. However, the modulated light intensity is an even function of the modulation voltage, and an electro-optical switch can be fully used only when the transmission direction of the analyzer is parallel or perpendicular to the polarization direction of the emerging light from the crystal. As for an electro-optical switcher based on such a crystal, the maximum modulation voltage required to achieve a full switching-state transition can be defined as its switching voltage. The π/4-voltage of a bismuth silicate (Bi12SiO20) crystal with dimensions of 6 mm×4 mm×2.9 mm is experimentally measured and found to be approximately 3 kV for a wavelength of 635 nm. Similarly, the concepts of π-stress and π-strain can be defined for the elasto-optic modulator with optical activity.