Bifunctional N,N'-dioxide catalysts have been developed for highly enantioselective cyanosilylation of alpha,alpha-dialkoxy ketones. This process, catalyzed by in-situ-prepared proline-derived N,N'-dioxide 2b, produced the corresponding cyanohydrin trimethylsilyl ethers in excellent yields (up to 99%) with high enantioselectivities (up to 93% ee). A reasonable mechanism was proposed according to the observation of the linear effect, 1H NMR spectra, isolated cyanohydrin, and the roles of the NH and N-oxide moieties of the catalyst.