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A high level ab initio study on electronic states of sulfur dimer S 2 is carried out by utilizing multi-reference configuration interaction plus Davidson correction (MRCI+Q) method. The modification to the electronic structures caused by spin-orbit coupling (SOC) effect is taken into consideration by the state-interacting method with the full Breit-Pauli Hamiltonian. The potential energy curves (PECs) of 19 Λ-S states and 52 Ω states generated from the Λ-S states are calculated. With the aid of calculated SOC matrix elements and the PECs of the Ω states, we discuss the predissociaiton of the B3Σ u − state. The variation of line width of B3Σ u −- X3Σ g − transitions with vibrational quantum number v ' of the B3Σ u − are determined from Fermi golden rule calculations. Our study indicate that the predissociation of the B3Σ u − state is induced by the strong spin-orbit coupling with different electronic excited states, resulting in the abnormal dependence of the dissociation rate on vibrational states. Unlabelled Image • Electronic states of S 2 are studied by MRCI+Q method including spin-orbit coupling effect. • Linewidths for B-X vibronic transitions are evaluated by using Fermi golden rule calculations. • Predissociation mechanism of the B3Σ − u state is analyzed. [ABSTRACT FROM AUTHOR]