Size control of the photo-electrochemical activity of a n-semiconductor oxide in water oxidation was demonstrated on size and shape controlled SrTiO3 nano-cubes featuring homogeneous {100} oriented surface. The carbonate free SrTiO3 perovskite nano-cubes were synthesized from titanium(IV) bis(ammonium lactato)dihydroxide solution and strontium nitrate in presence of the structure ordering gelatin. The gelatin content determines the size of the perovskite nano-cubes in the range 10-30 nm SrTiO3 nano-cubes show band gap in the range 3.2-3.3 eV with slight blue shift of the absorption edge for nanocubes smaller than 20 nm. All materials are active in photo-electrochemical water splitting, their specific activity, however, decreases with decreasing particle size and drop exponentially for nano-cubes smaller than 20 nm. (c) 2018 Elsevier Ltd. All rights reserved.