In addition to the dielectric relaxation around 170 K in the cubic structure of Srl_3x/2LaxTiO 3, a similar relaxation was observed at about 70 K in the tetragonal structure with an activation energy in the range 0.1 3-0.16 eV which increases as x in Srl_3x/2La,TiO 3 varies from 0.60-3.00 at %. This relaxation is explained by Skanavi's model and is discussed in terms of thermal motions of Ti 4+ between potential minima produced by lattice distortions in the tetragonal structure. In order to provide a direct evidence for the suggestion that the dielectric relaxation around 170 K in the cubic is due to thermal motions of Ti 4+, dielectric properties on manganese-doped specimens, Srl_3,/2La,MnyTil_y03 with x = 1.40 xl 0 -2 and y= 0.1 xl 0 -2, were investigated because Mn 4+ substitutes for Ti 4+. As well as the relaxation due to Ti 4+, this specimen exhibits another peak due to Mn 4+ with an activation energy somewhat smaller than that of Ti 4+. The activation energies and the relative intensity of these relaxation processes are explained by the difference in ionic radii of Mn 4+ and Ti 4+ and the difference in formation energies of a strontium vacancy adjacent to Mn 4+ and that to Ti 4+, which were calculated theoretically using a shell model.