The optimized geometry of 5-phenyltropolone (5PTRN) has been calculated by using the B3LYP/6-311+G(2d,p) method in the S-0 state. The seven-membered ring of the tropolone moiety deviates slightly from planarity. The equilibrium torsional dihedral angle in the So state is determined to be ca. 44 degrees, which is close to the experimental value for the isotopomer of 5PTRN, 5PTRN(-OD). There are two barriers for phenyl rotation at the torsional angles of 0 degrees and 90 degrees, in which the former barrier at 0 degrees is higher than the latter one at 90 degrees. The CIS calculation suggests that in the S-1 state, the deviation from planarity in the seven-membered ring of the tropolone moiety increases, and the barrier for phenyl rotation at the torsional angle of 0 degrees is lower than that at 90 degrees. Assignment of low-frequency bands is discussed based on the calculated vibrational fundamentals for the S-0 state.