Solvent effects on the selective hydrogenation of unsaturated aldehyde were studied by using three kinds of catalysts, Pd/C, Pt/C and Co/Al2O3. Many kinds of organic solvents, whose polarities were different, were used. Non-polar solvents had high selectivity to saturated aldehyde while polar solvents had high selectivity to unsaturated alcohol. The addition of water decreased the selectivity to saturated aldehyde.