Abstract: The adsorption and the mechanism for selective hydrogenation of cinnamaldehyde (CAL) on Pt (111) surface and Pt₁₄ cluster were investigated by using density functional theory (DFT). The results illustrate that the synergistic adsorption of CAL molecule on Pt (111) surface with C=O and C=C bonds is most stable at the Hcp position, whereas most stable adsorption of CAL appears on the Pt₁₄ cluster with C=C bond; the adsorption of CAL on the Pt₁₄ cluster is stronger than that on Pt(111) surface. The reaction barriers for each elementary reaction were determined from the transition state search and the results suggest that CAL was preferentially hydrogenated at C=O on the Pt(111) surface and Pt₁₄ cluster, forming cinnamyl alcohol (COL); the hydrogenation of O atom takes the priority. Both Pt plate and cluster have good selectivity for hydrogenation of CAL to COL. The reaction barrier of CAL hydrogenation on Pt(111) surface is lower than that on Pt₁₄ cluster, indicating that the catalytic activity and selectivity of CAL hydrogenation are closely related to the structure of Pt catalysts; Pt(111) surface is more favorable for catalyzing the hydrogenation of CAL to COL.