In this investigation we use B3LYP density functional theory (DFT) to investigate the CVD growth mechanism of (100) diamond. Our results are consistent with the Garrison mechanism in which the dimer-opening step involves simultaneous formation of a surface olefin and dissociation of the dimer. We calculate this step to have a barrier of 9.6 kcal/mol. The olefin is then attacked by a surface radical to form a six-membered ring. We find this reaction to be the rate-limiting step with an activation energy of 13.6 kcal/mol. This is in excellent agreement with the experimental value of 15 kcal/mol obtained by the selective growth method and XPS. The direct ring-opening and ring-closing reaction from adsorbed CH2 radical has an activation energy of 49.4 kcal/mol and does not contribute significantly to the growth rate. The barrier on larger clusters that include the effects of neighboring adsorbed hydrogen increases to 15.6 kcal/mol. Additionally, our calculated vibrational frequencies agree within 2% of experimental IR and HREELS spectra.