Molecular　dynamics　simulations　on　nanoindentation　of　circular　monolayer　molybdenum　disulfide　(MoS2)　film　are　carried　out　to　elucidate　the　deformation　and　failure　mechanisms.　Typical　force-deflection　curves　are　obtained,　and　in-plane　stiffness　of　MoS2　is　extracted　according　to　a　continuum　mechanics　model.　The　measured　in-plane　stiffness　of　monolayer　MoS2　is　about　182　+/-　14　Nm(-1),　corresponding　to　an　effective　Young＇s　modulus　of　280　+/-　21　GPa.　More　interestingly,　at　a　critical　indentation　depth,　the　loading　force　decreases　sharply　and　then　increases　again.　The　loading-unloading-　reloading　processes　at　different　initial　unloading　deflections　are　also　conducted　to　explain　the　phenomenon.　It　is　found　that　prior　to　the　critical　depth,　the　monolayer　MoS2　film　can　return　to　the　original　state　after　completely　unloading,　while　there　is　hysteresis　when　unloading　after　the　critical　depth　and　residual　deformation　exists　after　indenter　fully　retracted,　indicating　plasticity.　This　residual　deformation　is　found　to　be　caused　by　the　changed　lattice　structure　of　the　MoS2,　i.e.　a　phase　transformation.　The　critical　pressure　to　induce　the　phase　transformation　is　then　calculated　to　be　36　+/-　2　GPa,　consistent　with　other　studies.　Finally,　the　influences　of　temperature,　the　diameter　and　indentation　rate　of　MoS2　monolayer　on　the　mechanical　properties　are　also　investigated.