The factor dominating olefin to paraffin ratio in cobalt-catalyzed Fischer-Tropsch synthesis has been identified via experimental kinetic studies and theoretical calculations including the van der Waals interaction between the adsorbates and Co surfaces. The olefin to paraffin ratio for each carbon number is expressed as a function of adsorption energies of olefin and activation energies of hydrogenation steps. Density functional theory calculations on Co (0001) surface were performed to provide a comprehensive fundamental insight for the chemistry governing the olefin to paraffin ratio. The calculated olefin to paraffin ratio decreases with chain length except for ethylene, which agrees well with the experimental results. The differences introduced to the adsorption energies of olefin by van der Waals functional are correlated with d-band center changes of the Co surface. The carbon number dependent adsorption energies of olefin, rather than the activation energies of their hydrogenation reactions, are the driven force for the carbon number dependent olefin to paraffin ratio. The insights may delineate a new blueprint for the rational design of catalysts with enhanced olefin to paraffin ratio. (C) 2017 Published by Elsevier Inc.