Cubic boron nitride (cBN) is a kind of artificial (synthetic) crystal. The band gap of cBN crystal is about 6.3 eV, which has the zinc blende structure and the`43m symmetry. The cBN crystal has strong effects of frequency doubling and sum frequency generation which is second order nonlinear optical properties. The Nd: YAG laser with wavelength of 1064 nm is used. After the main beam passing KTP (KTiOPO4) which is a nonlinear optical crystal, the second harmonic with the wavelength of 532 nm is obtained. When the beam of 532 nm passes through the frequency doubling crystal cBN, it is frequency-doubled to 266 nm. Then the wavelength of 177 nm is obtained after the wavelength of 266 nm is frequency–summed with the wavelength of 532 nm. These light beams of 532, 266 and 177 nm come into the cBN crystal, they are absorbed. The photoconductance of unintentionally doped cBN is studied under the illumination of light rays with wavelength of 266 and 177 nm respectively. The electric signal is measured and analyzed.