Infrared (IR) absorption and polarized Raman spectra were measured in BaSi2 epitaxial films to investigate the vibrational modes and the symmetry of Si4 cluster in BaSi2. By an analysis based on Raman and/or IR activity in the spectra, the symmetry of Si4 cluster was determined as Th-symmetry and the observed Raman lines and IR peaks were assigned to Ag, Eg, Fg, and Fu, respectively. In the three Raman lines of Fg-mode, one LO phonon line and two TO phonon lines were classified by the depolarization ratio of polarized Raman intensities.