The optically active cobalt(III) complex with chiral cyclen, (2S,5S,B S,11S)-2,5,8,11-tetraethyl-1,4,7,10-tetraazacyclododecane, preferentially binds to D-phenylglycine (D-Phg) or D-t-leucine (D-t-Leu) rather than L-Phg or L-t-Leu, respectively, with 20% de in dimethyl sulfoxide at 293 K. Comparative studies on the crystal structures of cobalt(III) complexes with D-Phg and L-Phg revealed that the diastereo-selectivity is due to the difference in the steric hindrance that should occur between the amino group of Phg and the ethyl group of cyclen.