Abstract The photo-isomerization and photo-dimerization of cinnamic acid are discussed from the standpoint of the electronic structures in the ground and excited states. State energies, charge densities, bond orders and free valences are calculated by using the Pariser-Parr-Pople method, including the configuration interaction. The computed transition energies are in good agreement with the experimental results. It is found that the bond order between the α- and β-carbons of cinnamic acid decreases, while the free valences of the α- and β-carbons increases, upon the lowest electronic excitation. It is concluded that the photo-isomerization is due to the smaller bond order, and that the photo-dimerization is due to the larger free valences in the excited state of cinnamic acid.