Poly(O-acetyl, L-serine), which is a precursor of poly(L-serine) in synthesic sequence, is found to exist in cross-p conformation. Normal mode analyses including phonon dispersion have been performed, to understand completely the FTIR spectra of this polypeptide, using Wilson＇s GF matrix method and Urey-Bradley force held. The characteristic features of dispersion curves like repulsion, exchange of character and bunching have been observed and discussed. In addition to the above the heat capacity is calculated as a function of temperature via density-of-states derived from dispersion curves.