Electronic structures of nonlinear optical K_2Be_2BO_3F_2 crystals have been studied by means of DV-Xα method. It is found that the top region of valence band consists of mainly F-, B- and O-2p wave functions. The effective covalent bond charges are mostly localized on the B-O bonds, which is consistent with the fact that the (BO_3)^<3-> molecular ions play important role for the optical nonlinearity. The KBBF crystals are expected to be useful for the production of entangled photon pairs in UV-region by using spontaneous parametric up- and down conversions.