The pyrochlore-free 10 mol% Sm2O3-doped/undoped Pb(Mg1/3Nb2/3)O-3 ceramics were fabricated and their cationic ordering structures were investigated by X-ray/electron diffraction techniques. In the X-ray diffraction patterns, only fundamental diffraction peaks were observed for the undoped Pb(Mg1/3Nb2/3)O-3, while the (h + 1/2 k + 1/2 l + 1/2) superlattice reflections were also observed for the 10 mol% Sm2O3-doped Pb(Mg1/3Nb2/3)O-3. In the electron diffraction patterns, the (h + 1/2 k + 1/2 l + 1/2) superlattice reflections as well as the fundamental diffraction spots were observed for the undoped and the 10 mol% Sm2O3-doped Pb(Mg1/3Nb2/3)O-3, but the intensities of the superlattice reflections were stronger for the 10 mol% Sm2O3-doped Pb(Mg1/3Nb2/3)O-3. It has been experimentally assured that Sm-doping enhances the 1:1 ordering of Mg2+ and Nb5+ ions in Pb(Mg1/3Nb2/3)O-3.