The reaction mechanism of photochemical ring opening and closing transformation was investigated for diarylethene (DAE), which works as a molecular switch and photodevice. Spin-flip time-dependent density functional theory is employed to map the potential energy surfaces and to elucidate the photochemical mechanism of three isomers (normal, inverse, and mixed types) of 1,2-dithienylethene, a model DAE. The potential energy characteristics including the minimum-energy conical intersection reveals the origin of different product preferences of the three isomers. For the normal type, the excited state from either closed or open form reaches the same conical intersection that gives preferentially the closed product. The inverse type preferentially gives the closed product. The mixed type has two pathways that are easily convertible, and both open and closed reactants give both open and closed products.