The catalytic dehydrogenation of cyclohexane over transition metals is important in refining reforming as well as in hydrogen production strategies. Nanometer and subnanometer Pt particles demonstrate reasonably high selectivities, lower activation temperatures and higher efficiencies. Recent experimental results show that larger Pt particles (2.15 nm) selectively produce cyclohexene from cyclohexane; while smaller Pt particles (1.32 nm) result in the formation of benzene. We have used theory together with detailed characterization studies to elucidate the particle size effects on the selectivity. Density functional theory calculations were carried out using the Vienna ab-initio Simulation Package to obtain detailed insights into the mechanism of selective dehydrogenation of cyclohexane on platinum nano-particles. A cubo-octahedral Pt 55 cluster was employed to model the ∼1.3 nm platinum nano-particles. Larger platinum particles with diameter ∼2.0 nm were modeled by a truncated cubo-octahedral Pt 201 cluster. We examined in detail the reaction pathways for the dehydrogenation of cyclohexane over Pt 55 and Pt 201 clusters. The results show that surface species bind more strongly on the smaller cluster (Pt 55). The ability to activate the C-H bond increases in the order (111)-facet < (100)-facet < Edge < Corner, which correlates with the degree of coordinative unsaturation at these surface sites. The product selectivity towards cyclohexene vs. benzene was found to depend on the competitive partitioning between the C-H bond activation process and the desorption process of the cyclohexene intermediates. The computational results were found to be in good agreement with the experimental results.