Two-dimensional　(2D)　layered　materials　are　at　the　forefront　of　research　because　of　their　unique　structures　and　promising　catalytic　abilities.　Here,　the　structural　stability,　electronic　properties　and　gas　adsorption　of　metal　(V,　Nb,　Ta)-doped　monolayer　MoS2　have　been　investigated　by　density　functional　theory　calculations.　Our　results　show　that　the　metal　(V,　Nb,　Ta)-doped　monolayer　MoS2　is　a　stable　catalyst　under　room　temperature,　due　to　the　strong　interaction　between　the　doped　metals　(V,　Nb,　Ta)　and　S　vacancy　of　monolayer　MoS2.　Compared　with　the　gas　adsorption　(CO,　NO2,　H2O,　NH3)　on　pristine　monolayer　MoS2,　doped　metal　(V,　Nb,　Ta)　can　significantly　improve　the　adsorption　properties,　chemical　activity　and　the　sensitivity　of　that　of　adsorbed　gas　molecules.　This　effect　occurs　due　to　the　strong　overlap　between　the　metal　nd　orbitals　and　gas　molecule　orbitals,　result　in　activation　of　the　adsorbed　gas　molecules.　Analysis　of　Bader　charge　shows　that,　more　charge　transfer　(-0.66　e　to　-0.72　e)　occur　from　metal　(V,　Nb,　Ta)-doped　monolayer　MoS2　to　the　oxidizing　gas　molecules　(NO2)　acting　as　acceptors.　While　for　the　adsorption　of　CO　molecules,　the　relative　less　electrons　(about　-0.24　e　-　-0.35　e)　transfer　occuring　from　substrate　to　the　adsorbed　gases.　Whereas　the　direction　of　charge　transfers　is　reversed　for　the　adsorption　of　the　reducing　gas　(H2O　and　NH3)　behaving　as　donors,　in　which　small　electrons　(0.04　e　-0.09　e)　transfer　from　adsorbed　gas　to　metal　(V,　Nb,　Ta)-doped　monolayer　MoS2.　Our　results　suggested　that　metal　(V,　Nb,　Ta)-doped　monolayer　MoS2　might　be　a　good　candidate　for　low-cost,　highly　active,　and　stable　catalysts　and　gas　sensors,　providing　an　avenue　to　facilitate　the　design　of　high　active　MoS2-based　two　dimensional　catalysts　and　gas　sensors.　(C)　2017　Elsevier　B.V.　All　rights　reserved.