By using first-principles calculations, we investigated the monolayer ReS2 with vacancies under strain engineering, specifically focusing on its energy of formation, band gap, electron density of states, effective mass and optical properties. The calculated results disclose that S4 defect is more likely to form than other kinds of vacancies. Asymmetric deformation induced by strain makes its band structure transformation from direct band gap to indirect band gap. The analysis of the partial density of states indicates that the Re-d and S-p orbitals are the major components of the defect states, being different from MoS2, the defect states locate both above and below the Fermi level. Moreover, the effective mass was sensitive and anisotropic under the external strain. The reflection spectrum can be greatly tuned by the external strains, which indicates that the ReS2 monolayer has promising applications in nanoscale strain sensor and conductance-switch FETs. (C) 2017 Elsevier B.V. All rights reserved.