The formation energies of intrinsic vacancies in monolayer MoS2 as well as effect of the vacancies on electronic structures of monolayer MoS2 were investigated using the first-principles density functional theory. Results show that both Mo and S vacancies depend greatly on the Fermi level. With the increasing Fermi level, the formation energies of S vacancies increase whereas the formation energies of Mo vacancies decrease. Under reducing conditions, S vacancies are more likely to form and V-S(2+) is the dominant defect. In contrast, under oxidizing conditions, Mo vacancies are easier to form and V-Mo(4-) is the main source of defect. After introducing the intrinsic vacancies, the valence and conduction bands of monolayer MoS2 were expanded toward lower energy and the band gaps of monolayer MoS2 were decreased. Moreover, V-Mo(4-) brings about deep acceptor-like levels and p-type conductivities, whereas V-S(2+) induces deep donor-like levels and n-type conductivities. (C) 2014 Elsevier B.V. All rights reserved.