The optimized molecular structures of the cis and trans rotamers of m-fluorostyrene in the ground S 0 and excited S 1 states are searched by ab initio and density functional theory (DFT) calculations. Their vibrational spectra in the S 1 state are studied by one color resonant two photon ionization (1C-R2PI) method. In consideration of the optimized structural geometries, the interaction of the vinyl group with the ring is stronger in the S 1 state than that in the S 0 state, and the C1–Cα bond exhibits a partial double bond character in the S 1 state. Most of the active vibrations assigned from the R2PI spectrum are found to be the in-plane ring modes. Analysis on the vibational frequencies shows that there is an increase for the C1–Cα stretching, and a decrease for the CC stretching of the vinyl group in the S 1 state with respect to those in the S 0 state.