Monolayer　molybdenum　disulfide　(MoS2)　has　obtained　much　attention　recently　and　is　expected　to　be　widely　used　in　flexible　electronic　devices.　Due　to　inevitable　bending　in　flexible　electronic　devices,　the　structural　and　electronic　properties　would　be　influenced　by　tensile　strains.　Based　on　the　density　functional　theory　(DFT),　the　structural　and　electronic　properties　of　monolayer　MoS2　with　a　sulfur　(S)-vacancy　is　investigated　by　using　first-principles　calculations　under　uniaxial　tensile　strain　loading.　According　to　the　calculations　of　vacancy　formation　energy,　two　types　of　S-vacancies,　including　one-sulfur　and　two-sulfur　vacancies,　are　discussed　in　this　paper.　Structural　analysis　results　indicate　that　the　existence　of　S-vacancies　will　lead　to　a　slightly　inward　relaxation　of　the　structure,　which　is　also　verified　by　exploring　the　change　of　charge　density　of　the　Mo　layer　and　the　decrease　of　Young’s　modulus,　as　well　as　the　ultimate　strength　of　monolayer　MoS2.　Through　uniaxial　tensile　strain　loading,　the　simulation　results　show　that　the　band　gap　of　monolayer　MoS2　decreases　with　increased　strain　despite　the　sulfur　vacancy　type　and　the　uniaxial　tensile　orientation.　Based　on　the　electronic　analysis,　the　band　gap　change　can　be　attributed　to　the　π　bond-like　interaction　between　the　interlayers,　which　is　very　sensitive　to　the　tensile　strain.　In　addition,　the　strain-induced　density　of　states　(DOS)　of　the　Mo-d　orbital　and　the　S-p　orbital　are　analyzed　to　explain　the　strain　effect　on　the　band　gap.　©　2018　by　the　authors.　Licensee　MDPI,　Basel,　Switzerland.