The　microstructure　observation　and　mechanical　property　test　were　used　to　analyze　welded　joints　of　main　steam　pipe　elbow(P91　steel)　after　about　32　000　h　in　service　from　a　supercritical　unit　in　a　thermal　power　plant.　The　results　show　that　circumferential　cracks　were　found　at　junction　between　weld　and　base　metal　on　the　inner　surface　of　the　elbow　after　a　long　period　of　service.　The　mechanical　properties　such　as　tensile　strength,　hardness　and　impact　energy　of　main　steam　pipe　material　met　the　requirements　of　relevant　standards,　but　the　microstructure　had　creep　damage.　The　main　reason　for　equipment　leakage　was　due　to　different　mechanical　properties　of　welded　joints　with　different　materials,　and　then　the　pipe　initiated　creep　deformation　under　high　temperature　leading　to　heat　affected　zone　cracking.