Infrared absorption spectra of naphthalene in supercritical carbon dioxide (CO2) solutions of various concentrations were measured at 318.15 K at various pressures. The molar absorption coefficient of naphthalene in supercritical CO2 Was found to be dependent on the density Of CO2. As die density Of CO2 increased, a red shift of peak wavenumber of the C-C vibration (quadrant stretching) were observed. This suggests that the molecular vibration of naphthalene in supercritical CO2 was affected by the fluid density.