ABSTRACT We perform density functional theory method to study the electronic structure of relaxor ferroelectrics Pb(Mg1/3Nb2/3)O3. An ordered perovskite structure based supercell was examined with the B sites Nb-Mg-Nb stacking along the [001] direction. Band structure, density of states and electron density have been obtained. The calculated band gap is a direct one with the value of 1.05 eV. It is found that the Mg-O bond presents no covalency, whereas there is a considerable Nb-O covalent bonding. The different B-O bonding characters might be responsible for the relaxor properties of Pb(Mg1/3Nb2/3)O3 crystal.