Carbon-supported Pt catalysts prepared by the impregnation method in water - dimethyl sulfoxide (DMSO) solutions were investigated. Using X-ray diffraction (XRD) technique for estimation of the particle size and shapes, we show that morphology of the Pt/C catalysts depends strongly on the DMSO content in the solutions. The average particles size reduces and a shape of the Pt nanocrystals changes from a truncated octahedron to a cuboctahedron, and eventually to a truncated cube with increasing DMSO concentration in the reaction mixture. Antibate effects of the size and shape cause non-monotonic dependence of the catalytic activity for the oxygen reduction reaction (ORR) with electric current maximum at an average particle size of 2.7nm.