Single phase Pb(Mg 1/3 Nb 2/3 )O 3 ceramics doped by 10 mol% of electron donors such as La 3 ,Pr 3+,4+ ,Nd 3+ ,Sm 3+ , were synthesized and their B-site cationic ordering structures were investigated by XRD and TEM. In the XRD patterns, only fundamental reflections were observed for the undoped Pb(Mg 1/3 Nb 2/3 )O 3 , while the (h/2 \textsck /2ι/2)(h, \textsck ,ι all odd) superlattice reflections resulting from the 1:1 ordering induced unit cell doubling were also observed for the donor-doped Pb(Mg 1/3 Nb 2/3 )O 3 . In the TEM selected area diffraction patterns, the (h/2 k/2 l/2)(h,k,l all odd) superlattice reflection spots as well as the fundamental reflection spots were observed for all the samples, but the relative intensities of the superlattice reflection spots to the fundamental reflection spots were significantly enhanced by the donor-doping. In the TEM dark-field images, antiphase boundaries were observed only for the donor-doped Pb(Mg 1/3 Nb 2/3 )O 3 . It was therefore experimentally verified that doping by electron donors such as La 3 ,Pr 3+,4+ ,Nd 3+ ,Sm 3+ , enhances the B-site cationic 1:1 ordering in Pb(Mg 1/3 Nb 2/3 )O 3 . These experimental results were interpreted in terms of the charge compensation mechanism.