Phase　stability　of　nanocrystals　and　its　targeted　transition　is　of　great　significance　for　the　controllable　synthesis　of　nanocrystals　with　particular　properties　and　uitilizations.　Crystal　size　and　surface　environment　are　generally　regarded　as　two　critical　factors　with　the　relative　phase　stability,　whereas　the　thermodynamic　realtionship　between　them　are　still　unclear.　Herein,　we　utilized　MnS　nanocystal　of　9-nm　as　a　probe,　to　study　the　phase　stability　and　transition　of　nanocrystals　under　different　surface　environments,　one　of　which　with　a　strong　surfactant　of　mercaptoethanol　and　the　other　with　pure　H2O,　respectively.　Combining　analysis　including　X-ray　diffraction,　transmission　electron　microscopy　and　scanning　electron　microscopy　indicated　that　the　critical　size　of　MnS　transforming　from　wurtzite　phase　to　rock-salt　phase　was　similar　to　200　nm　in　pure　H2O.　However,　this　value　decreased　to　13　nm　in　mercaptoethanol,　which　was　more　than　15　times　smaller　than　that　in　pure　H2O.　Thermodynamic　analysis　further　demonstrated　a　decreasing　trend　of　critical　size　for　phase　transformation　when　the　surface　of　nano　MnS　was　covered　by　strong　surfactant,　which　was　caused　by　the　decreasing　of　surface　free　energy.　This　is　probably　due　to　more　active　coordination　sites　of　rock-salt　MnS　owing　to　octahedral　configuration　of　Mn　with　S.　This　study　provided　a　critical　evidence　to　prove　the　fundamental　influence　of　surface　environment　on　phase　stability,　which　may　provide　a　novel　thought　on　regulating　phase　transition　of　crystals　in　nanoscale.