Novel optically active phenylacetylenes having diketopiperizine were synthesized from L-phenylalanine and polymerized with a rhodium catalyst to obtain the polymers with number-average molecular weights over 10 000 in good yields. The CD and UV - vis spectra of the polymers indicated that they took helical structures with predominantly one-handed screw sense in DMF, while random coil Structures in CHCl3. Addition of various Carboxylic acids such as trifluoroacetic acid to the polymer solution in CHCl3 promoted the helical formation. Molecular mechanics calculation suggested that the most stable conformer was a right-handed helix accompanying tandem hydrogen-bonding strands between the amide groups.