The triplet 3- thienylcarbene and α - thial - methylenecyclopropene were observed as the main products upon photolysis ( λ > 540 nm) of 3- thienyldiazomethane (3-TD) under the matrix-isolated conditions. The underlying mechanism was explored by the combined CASPT 2 and CASSCF calculation in the present work. The S1 , T2 , and T1 surfaces were found to be quasi-degenerate over a wide structural region. Meanwhile, there is a relatively strong spin – orbit interaction between the S 1 and T 2 states, due to the sulfur-atom effect. As a result, the triplet 3- thienylcarbene can be formed with a noticeable effi - ciency. The S1 minimum and S1 /S0 intersection structures for 3-TD were predicted to be undistinguishable by the present CASPT 2// CASSCF calculations. Therefore, the S1 → S0 internal conversion that occurs in the vicinity of the S1 /S0 intersection (S1 minimum) is in competition with the S1 → T1 intersystem crossing through the three-surface (S1 , T2 , and T1 ) intersection region. Once the excited 3-TD molecule returns to the S 0 state, the singlet 3- thienylcarbene can be generated with a considerable effi ciency, which is followed by the carbene rearrangement, leading to the fi nal product of α - thial-methylenecyclopropene ( α - TMC ). The concerted N2 loss and rearrangement in the excited state was proposed for the α - TMC formation in previous experimental study, which is not supported by the present CASPT 2// CASSCF calculation. The reason for this might come from the matrix effect, which is open for further study.