The potential energy surface of the reaction of SiH2 and C2H4 has been calculated by means of ab initio molecular orbital theory at the QCISD(T)/6-311G(d,p)//MP2/6-31G(d,p)+ZPE level. The barriers thus calculated for the various reaction channels were utilized for the calculation of the rate constants in the framework of quantum statistical Rice-Ramperger-Kassel theory. The contributions of the individual reaction channels towards the total rate constant have been examined. The calculated rate constants reveal that the disappearance of the reactants at all pressures is due to the formation of silirane (cyclic CH2CH2SiH2). The formation of silirane is always important within the temperature range 300-700 K and at pressures of 1-100 Torr. The calculations show that formation of vinylsilane (H2C=CH-SiH3) is not possible within the temperature range 300-700 K. It is suggested, based on these calculations, that vinylsilane cannot be formed during the thermal decomposition of ethylsilane at 723 K. The experimentally detected trace amount of product, along with the primary products SiH2, and C2H4, during the thermal decomposition of ethylsilane could be 2-methyl-1-silaethylene (H2Si=CHCH3).