Poly(L-leucine) is one of the poly(amino acids) having a bulky hydrophobic side chain. For want of full phonon dispersion curves and density-of-states on this biopolymer Roles ct al. have interpreted their heat capacity data in a limited way. In the present paper, we report the Fourier transform infra-red (FTi.r.) spectra and an analysis of the normal modes and their dispersion based on the calculations for an infinite chain and Urey Bradley force field with intrachain interactions only. The results thus obtained agree well with our FTi.r. spectra and the Raman frequencies reported by Koenig er nl. Several earlier assignments have been revised. A special feature of some dispersion curves is their tendency to bunch in the neighbourhood of the helix angle. This has been attributed to the presence of strong intramolecular coupling between different types of motions. Repulsion between the dispersion curves is also observed. The heat capacity obtained from the dispersion curves via density-of-states is in very good agreement with the experimental measurements beyond 220 K. It is observed that the main contribution to heat capacity comes from the modes involving the coupling of the backbone skeletal and side-chain motions.