Abstract: The mechanism and potential energy surface for the addition reactions of two germasilenes (H₂Ge=SiH₂ and Ph₂Ge=SiPh₂) with H₂O were investigated by using density functional theory (DFT), and the origin of the regioselectivity for addition reactions was analyzed. The results indicated that the monomer, dimer or trimer of H₂O may react with germasilenes as nucleophilic reagent. Both the reactions involving a forming Si—O or Ge—O bond are complicated reactions, and the formation of Si(Ge)—O bond precedes that of Ge(Si)—H bond. The regioselectivity of addition reactions is decided by kinetic factor. Behaving as a nucleophilic reagent, H₂O is less reactive toward germasilene than CH₃OH.