The transverse electro-optic (EO) modulation system is built based on cubic boron nitride (cBN) single crystals unintentionally doped and synthesized at a high pressure and high temperature. The photoelectric output of the system includes two parts that can be measured respectively and the value of elements in the linear EO tensor of the cBN crystal can be obtained. This method does not need to measure the absolute light intensity. All of the surfaces of the tiny cBN crystals whose hardness is next to the hardest diamonds are {111} planes. The rectangular parallelepiped cBN samples are obtained by cleaving along {110} planes and subsequently grinding and polishing f112g planes of the tiny octahedral cBN flakes. Three identical non-zero elements of the EO tensor of the cBN crystal are measured via two sample configurations, and the measured results are very close, about 3.68 and 3.95 pm/V, respectively, which are larger than the linear EO coefficients of the general III-V compounds.