The activation mechanisms of a methane molecule on a Pt atom (CH4-Pt) and on a Pt tetramer (CH4-Pt-4) were investigated using density functional theory (B3LYP and PW91) calculations. The results from these two functionals are different mostly in predicting the reaction barrier, in particular for the CH4-Pt system. A new lower energy pathway was identified for the CH4 dehydrogenation on a Pt atom. In the new pathway, the PtCH2 + H-2 products were formed via a transition state, in which the Pt atom forms a complex with carbene and both dissociated hydrogen atoms. We report here the first theoretical study of methane activation on a Pt-4 cluster. Among the five single steps toward dehydrogenation, our results show that the rate-limiting step is the third step, that is, breaking the second C-H bond, which requires overcoming an energy barrier of 28 kcal/mol. On the other hand, the cleavage of the first C-H bond, that is, the first reaction step, requires overcoming an energy barrier of 4 kcal/mol.