We have studied the reaction of allylsilanes with aldehydes by applying the ab initio MO theory. It has been shown that the reaction takes place via a transition state involving pentacoordinated silicon species, The oxygen of aldehyde attacks an apical site of the silicon center, while the allyl group departs directly from an equatorial site without causing a pseudorotation. The calculation has shown that the reaction of an allylsilacyclobutane model with formaldehyde has a lower activation barrier than the reaction of allylsilane or methyl-substituted allylsilanes. The barrier height is shown to be correlated well with the angle of coordination of substituents on the silicon center. The reactivity of allylsilanes against nucleophiles is discussed by evaluating the local acidic strength of the reaction site.