We perform periodic Density Functional Theory (DFT) calculations to study adsorption of benzene on a Pt (111) surface. van der Waals (vdW) interaction is included using vdW-DF functionals. Benzene is found to chemisorb strongly, and we obtain adsorption energy and geometry at all possible unique orientations on Pt (111) surface which agree well with experiments and previous calculations. We also find that while vdW interaction is crucial to the energetics of adsorption of benzene on Pt (111), the equilibrium geometry of the adsorbed molecule and its electronic structure are not so sensitive to this interaction.