The mechanisms for reaction between bis(cyclopentadienyl)magnesium and Si(10 0)-(2 x 1) surface are investigated with the aid of density functional theory calculations, The reactions on hydrogenated and hydroxylated Si surfaces are compared to understand the dominated initial reaction during atomic layer deposition of MgO on Si surface. The overall reaction energy and activation barrier are calculated for each reaction. It is found that the reaction of bis(cyclopentadienyl)magnesium with OH-terminated Si surface is both kinetically and thermodynamically more favorable than that with H-terminated Si surface.