The mechanisms of photoisomerization reactions were investigated theoretically using a model system of 2-methylfuran with the CASSCF (10-electron/8-orbital active space) and MP2-CAS methods and the 6-311-(d,p) basis set. After 2-methylfuran molecules are produced in the T, state by photoexcitation at 254 rim, intersystem crossing to the So surface is the most probable pathway for deactivation. Relaxing to the So state, the 2-methylfuran molecules can dissociate into 3-methylcyclopropene and carbon monoxide products. Otherwise, they may revert to singlet 2-methylfuran or undergo photorearrangement to produce 3-methylfuran. These stepwise mechanisms are consistent with the available experimental observations.