Abstract: Effect of long term thermal aging on microstructure and mechanical properties of P92 steel weld joints were studied by optical microscope（OM）,scanning electron microscope（SEM）,transmission electron microscope（TEM） and mechanical properties test.Results showed that the tensile strength of P92 weld joints decrease slowly even the aging time reached 7 000 h at 650 ℃.At initial aging stage（before 1 000 h） impact toughness of P92 weld metal decrease obviously,which keep at about 25J after aging for 1 000 h.The steady martensite matrix and the slow growth of M23C6 and MX should be responsible for the tensile strength maintenance of P92 weld joints.Aging embrittlement of weld metal results from coarse Laves phase precipitated along the prior austenite grain boundaries and martensite packets.