Ab initio molecular orbital calculations were used to examine the Diels-Alder reaction of s-cis-1,3-butadiene on diamond surface. It was found that the activation energy on the surface is significantly decreased and exothermicity is significantly increased because of the reduced overlap between the p orbitals of the surface C=C dimer. This enhancement of the reactivity on the surface is larger than that produced by chemical substitution of maleic anhydride for ethylene. Calculations show that dienes that contain heteroatoms such as N or S also cause the Diels-Alder reactions easily on the C(001)-(2x1) surface.