Abstract Ultra-soft pseudopotential within the local density approximation was used to investigate the geometry and electronic structures of para-electric solid solution KTa 1/2Nb 1/2O 3 (KTN) crystal. Three arrangements of B site cations (three modes) were presented. The geometry structures, band structures, and density of states were calculated. The difference of the calculated properties were compared among the three modes, and it is found that BO 6 octahedron and K–O bond lengths as well as partial density of states of O atoms are significantly different for three modes. Hence, it is proved that the effect of B site cations influences the geometry and electronic structure of para-electric KTN evidently.