Electronic structures of nonlinear optical Sr_2Be_2B_2O_7 crystals have been studied by means of DV Xa method. It is found that the top region of valence band consists of mainly B- and O-2p wave functions, while 2s and 2p wave functions of Be and B atoms contribute to the conduction band. The effective covalent bond charges are mostly localized on the B-O bonds which indicates that the (BO_3)^<3-> molecular ring plays an important role for the optical nonlinearity. From the mapping of the electron distributions, it is found that easy cleavage planes are expected to be parallel to the (ab)-plane.