The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system, bicyclo[4,1,0]hept-3-ene (1), with the CASSCF (six-electron/six-orbital active space) and MP2-CAS methods and the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such phototranspositions, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for bicyclo[4,1,0]hept-3-ene is as follows: reactant -> Franck-Condon region -> conical intersection -> intermediate -> transition state -> photoproduct. Two reaction paths, which lead to final photoproducts, have been identified: (path I) ring expansion and (path II) ring closure. The former is more favorable than the latter. Also, our theoretical findings strongly indicate that there is a substantial interaction between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of 1.