Refractive index, birefringence, and linear electro‐optic (EO) effect of BO6‐type oxides have been analyzed using a two‐term Sellmeier dispersion relation. Two oscillators correspond to two main transitions; the transition tod&egr; conduction band with a transition energy of 4–6 eV and that to thed&ggr; band with an energy of 9–11 eV. It is found that the contribution of the latterd&ggr; transition to the refractive index in the visible to the infrared region is larger than that of the former, and that the dispersion energyEidispfor thed&ggr; transition is approximately equal to that for the average single oscillator. The birefringence and the linear EO effect are explainable by the polarization‐induced changes in the position and the strength of thed&egr; transition. In the ferroelectric phase, the change in the oscillator position is found to be proportional to the square of the spontaneous polarizationPs2. On the other hand, the change in the oscillator strength consists of the terms proportional toPs2andPs4. The second term leads directly to two observed features: the quadratic EO coefficient depends onPs; and a slight difference exists between the coefficients derived from the spontaneous birefringence and the linear EO effect.