This work deals with the theoretical study on the molecular structure and vibrational spectra of two well known narcotics- Codeine and Morphine. The spectra were interpreted with the aid of normal mode analysis following full structure optimization based on the density functional theory and 6-311G + (d, p) basis set combination. A detailed interpretation of the infrared spectra of Morphine and codeine are reported in the present work. The similarities and differences between the vibrational analysis of the two molecules studied have been highlighted. The scaled theoretical wave numbers are in perfect agreement with the experimental values. The thermodynamic calculations related to the title compounds were also performed at B3LYP/6- 311G + (d, p) level of theory. The FT-Raman and FTIR spectra of Morphine have been taken from the literature.