Insertion reactions of C2H4 into the Pt-H and Pt-SiH3 bonds of a platinum(II) hydride silyl ethene complex, PtH(SiH3)(PH3)(C2H4), are investigated with the ab initio MO/MP4SDQ method. When the H ligand lies at the position trans to C2H4, C2H4 is inserted into the Pt-SiH3 bond with a significantly high activation energy (E(a)) of 54 kcal/mol. When SiH3 lies at the position trans to C2H4, C2H4 is inserted into the Pt-H bond with much lower E(a) (21 kcal/mol). Although the product just after the insertion involves a strong Pt-H-beta agostic interaction, deinsertion of C2H4 occurs with nearly no barrier. To complete hydrosilation of ethene, further conversion to the most stable product must occur by breaking the agostic interaction of about 17 kcal/mol. The total E(a) of 38 (21 + 17) kcal/mol is lower than the E(a) of the above-mentioned C2H4 insertion into the Pt-SiH3 bond. When PH3 is at the position trans to C2H4, C2H4 is inserted into the Pt-SiH3 bond with E(a) of 16.2 kcal/mol and into the Pt-H bond with E(a) of only 4.4 kcal/mol. These results lead to the conclusions that ethene is much more easily inserted into the Pt-H bond than it is into the Pt-SiH3 bond and that the Pt-catalyzed hydrosilation of alkene proceeds through the Chalk-Harrod mechanism. Determining factors for the ease of the insertion are the Si-C and C-H bond energies, the trans-influence of the ligand at the position trans to C2H4, the directionality of valence or orbitals of H and SiH3, and sometimes the agostic interaction between Pt and the C2H5 group formed in the reaction.