The mechanisms for the photoisomerization reactions are studied theoretically at the M06-2X/Def2-TZVPD level of theory, using the five-coordinated M(CO)(4)(CS) (M = Cr, Mo, and W) complexes as model systems. This study provides the first theoretical evidence for the mechanisms of these photorearrangements of the five coordinated metal complexes. That is, the photoisomerization process is primary the axial to basal movement of CS. The model study demonstrates that the preferred reaction route for the photorearrangement reactions is as follows: reactant -> FranckCondon region -> minimum (triplet) -> transition state (triplet) -> triplet/singlet intersystem crossing -> photoproduct. The theoretical results also show that the energy differences between the crucial points are quite small, which demonstrates that the CS group rotates easily to form the different conformations when the M(CO)4(CS) molecules have been photoirradiated. These photochemical mechanisms are consistent with the available experimental observations.