The reaction mechanism of the l-prolinecatalyzed α-aminoxylation reaction between aldehyde and nitrosobenzene has been investigated using density functional theory (DFT) calculation. Our calculation results reveal following conclusions [1]. The first step that corresponds to the formation of C-O bond, is the stereocontrolling and rate-determining step [2]. Among four reaction channels, the syn-attack reaction channel is more favorable than that of the anti one, and the TS-ss channel dominates among the four channels for this reaction in the step of C-O bond formation [3]. The intermolecular hydrogen bond between the acidic hydrogen of l-proline and the N atom of the nitrosobenzene in an early stage of the process catalyzes very effectively the C-O bond formation by a large stabilization of the negative charge that is developing at the O atom along the electrophilic attack [4]. The effect of solvent decreases the activation energy, and also, the calculated energy barriers are decrease with the enhancement of dielectric constants for C-O bond formation step. These results are in good agreement with experiment, and allow us to explain the origin of the catalysis and stereoselectivity for l-proline-catalyzed α-aminoxylation of aldehyde reaction. The addition of H 2 O to substituted imine proline, intermolecular proton-transfer steps, and the l-proline elimination process were also studied in this paper.