Blue phases generally appear at narrow temperature range between chiral nematic phase and Iso phase. There are three types of blue phases, BPI, BPII and BPIII with increasing temperature, their structures are different. BPI and BPII are body centered cubic and simple cubic, respectively, but the structure of BPIII is not yet clear. Because of cubic lattice, blue phases are optically isotropic and have chiral pitches, which are related to the lattice constants of the three-dimensional structures. Electro-optic properties of blue phases are well explained by Kerr effect. Kerr constant, K, was known as a dominant parameter of Kerr effect. In this study, we present the relation between electro-optic properties and chiral pitches.