The Cp2Zr-catalyzed hydrosilylation of ethylene was theoretically investigated with DFT and MP2-MP4(SDQ) methods, to clarify the reaction mechanism and the characteristic features of this reaction. Although ethylene insertion into the Zr-SiH3 bond of Cp2Zr(H)(SiH3) needs a very large activation barrier of 41.0 (42.3) kcal/mol, ethylene is easily inserted into the Zr-H bond with a very small activation barrier of 2.1 (2.8) kcal/mol, where the activation barrier and the energy of reaction calculated with the DFT-(B3LYP) method are given and in parentheses are those values which have been corrected for the zero-point energy, hereafter. Not only this ethylene insertion reaction but also the coupling reaction between Cp2Zr(C2H4) and SiH4 easily takes place to afford Cp2Zr(H)(CH2CH2SiH3) and Cp2Zr(CH2CH3)(SiH3) with activation barriers of 0.3 (0.7) and 5.0 (5.4) kcal/mol, respectively. This coupling reaction involves a new type of Si-H a-bond activation which is similar to metathesis. The important interaction in the coupling reaction is the bonding overlap between the d(pi)-pi* bonding orbital of Cp2Zr(C2H4) and the Si-H sigma* orbital. The final step is neither direct C-H nor Si-C reductive elimination, because both reductive eliminations occur with a very large activation barrier and significantly large endothermicity. This is because the orbital Of Cp2Zr is at a high energy. On the other hand, ethylene-assisted C-H reductive elimination easily occurs with a small activation barrier, 5.0 (7.5) kcal/mol, and considerably large exothermicity, -10.6 (-7.1) kcal/ mol. Also, ethylene-assisted Si-C reductive elimination and metatheses of Cp2Zr(H)(CH2CH2SiH3) and Cp2Zr(CH2CH3)(SiH3) with SiH4 take place with moderate activation barriers, 26.5 (30.7), 18.4 (20.5), and 28.3 (31.5) kcal/mol, respectively. From these results, it is clearly concluded that the most favorable catalytic cycle of the Cp2Zr-catalyzed hydrosilylation of ethylene consists of the coupling reaction of Cp2Zr(C2H4) with SiH4 followed by the ethylene-assisted C-H reductive elimination.