Periodically poled crystals of stoichiometric lithium tantalate doped with MgO (PP-Mg:SLT) were developed for a high-power optical frequency conversion. Inverted domain structures with 7.7 μm period were fabricated in 1-mm-thick crystals by an electric-field poling using patterned metallic electrodes. Frequency-doubling characteristics of these units were investigated with continuous and pulsed Nd: YAG lasers. The nonlinear coefficient, d33, of Mg:SLT derived from these measurements is 13 pm/V, similar to that of the congruent lithium tantalate. A stable generation of 5.5 W average power of green light at 532 nm was obtained while the photoinduced breakdown of the periodically poled KTiOPO4 was observed under similar experimental conditions. These results demonstrate the advantage of Mg:SLT for high-average-power applications.