The effects of two different amino acid catalysts on the stereoselectivities in the direct Mannich reactions of cyclohexanone,p-anisidine and p-nitrobenzaldehyde were studied with the aid of density functional theory.Transition states of the stereo-determining C “C bond-forming step with the addition of enamine intermediate to the imine for the L-proline(·-amino acid) and (R)-3-pyrrolidinecarboxylic acid(·-amino acid)-catalyzed processes were reported.B3LYP/6-31G calculations provide a good explanation for the opposite syn vs.anti diastereoselectivities of these two different kinds of catalysts(syn-selectivity for the ·-amino acid catalysts,anti-selectivity for the ·-amino acid catalysts).Calculated and observed diastereomeric ratio and enantiomeric excess values are in reasonable agreement. The effects of two different amino acid catalysts on the stereosectivity in the direct Mannich reactions of cyclohexanone, p-anisidine and p-nitrobenzaldehyde were studied with the aid of density functional theory. Transition states of the stereo-determining C ”C bond-forming step with the addition of enamine intermediate to the imine for the L-proline (-amino acid) and (R) -3-pyrrolidinecarboxylic acid (-amino acid) -catalyzed processes were reported. B3LYP / 6-31G calculations provide a good explanation for the opposite syn vs.anti diastereoselectivities of these two different kinds of catalysts (syn-selectivity for the -amino acid catalysts, anti-selectivity for the -amino acid catalysts). Calculated and observed diastereomeric ratios and enantiomeric excess values are in reasonable agreement