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We investigated photocatalytic property of two dimensional heterostructure MoS 2 /WS 2 by a first principles calculation. Firstly, we evaluated the energetic stability of different heterostructures by comparing combination energies of heterojunctions with those between adjacent layers in multilayer MoS 2 or WS 2 system after constructing nine heterostructures (MoS 2) n /(WS 2) m (n, m = 1, 2, 3). The subscripts n and m are the numbers of MoS 2 or WS 2 layers that are contained in the above heterostructures, respectively. The investigated heterostructures have all stable binding energy of about 0.2 eV. Secondly, we calculated band gap of every heterostructure in order to compare it with the oxidation/reduction level for water splitting. Among the considered nine structures, the heterostructure (MoS 2) 1 /(WS 2) 1 is suitable for overall water splitting because it has a band gap of 1.70 eV and energy barrier of 0.21 eV for preventing recombination of electrons and holes generated by light irradiation. In addition, the heterostructure is also favorable to the evolution of hydrogen, as it has a low hydrogen desorption energy of 0.035 eV. • The considered heterojunctions are all stable energetically. • Monolayer WS 2 contributes to overall water-splitting. • Multilayer WS 2 contributes to half water-splitting. • (MoS 2) 1 /(WS 2) 1 is favorable to separating electrons and holes. • (MoS 2) 1 /(WS 2) 1 has a good hydrogen desorption property. [ABSTRACT FROM AUTHOR]