The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of bicyclo[4, 1,0]hept-2-ene (2-norcarene) 1 with the CASSCF (eight-electron/eight-orbital active space) and MP2-CAS methods and the 6-311 (d) basis set. The structures of the conical intersections and intersystem crossings, which play a crucial role in such photoisomerization reactions, 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 singlet photoreaction route for 1 is as follows: singlet reactant -> Franck-Condon region -> conical intersection -> intermediate -> transition state -> photoproduct. On the other hand, our theoretical findings indicate that the preferred triplet photoreaction route for 1 is as follows: singlet reactant -> Franck-Condon region -> triplet minimum -> triplet transition state -> intersystem crossing -> intermediate -> singlet transition state -> photoproduct. In particular, the intersystem crossing mechanism found in this work gives a better explanation and supports the available experimental observations. Two kinds of reaction pathways, which can lead to final photoproducts, have been identified: (paths I or III ring-expansion to form a cycloheptene ring and (paths II or IV) ring-closure to form a methylcyclohexene structure. Both exhibit biradical character. Also, our theoretical investigations strongly indicate that substantial interaction occurs between the cyclopropane moiety and the isolated carbon-carbon double bond in the excited state of (1).