The transverse‐optical (TO) phonon mode in a CdS film with a thickness less than 410 A˚ is observed at 4880 A˚ excitation wavelength which is above the band gap of CdS bulk (2.42 eV) at room temperature. This phenomenon is ascribed to the size quantization of the free carrier in the low‐dimensional thin‐film structure. The quantum size effect causes a blue shift of the band gap in the as‐deposited CdS thin film. The Raman shift of the TO mode of CdS film is around 220 cm−1. The softening energy of the TO phonon mode is about 8 cm−1. It was found that this softening energy is independent of the film thickness.