Effects of Pd particle size and support acidity on enantioselective hydrogenation of α,β-unsaturated carboxylic acids were systematically studied using Pd nanoparticles with different size immobilized on various oxides. Small Pd particles showed higher activity in the hydrogenation of (E)-2-methyl-2-pentenoic acid due to the larger fraction of edge sites which were more active in olefin hydrogenation; but they did not change the reaction mechanism and/or the adsorption mode of reaction intermediates. Similar correlations in the hydrogenation of (E)-2-methyl-2-butenoic acid and 2-acetamidoacrylic acid further confirmed that the size of Pd particles only mediated the activity but did not alter the enantioselectivity. On the other hand, the activity and the enantioselectivity were strongly dependent on the acidity of the support. The TOF and the ee value followed the order TiO 2 ＞γ-Al 2 O 3 ＞SiO 2 ＞CeO 2 , suggesting that the acidic support favored the adsorption of the reaction intermediates.