DFT/6-31G* calculations were applied to study the direct aldol reaction between acetone and 4-nitrobenzaldehyde catalyzed by (S)-proline. Four transition states associated with the stereo-controlling step, corresponding to syn and anti arrangements of methylene moiety related to the carboxylic acid group in enamine intermediate and re and si attacks to the aldehyde carbonyl carbon have been obtained. The solvent effect of DMSO was investigated with polarized continuum model. The computed energies of transition states reveal the stereo-selectivity of the reaction.