Abstract: The uniaxial creep tensile strength, fracture features and microstructures of friction stir welded joint with postweld heat treatment for CLAM steel have been investigated in the range of the creep applied stress from 180 MPa to 300 MPa at 823 K condition. It is found that the creep life of the FSW joints of CLAM steel increase from 1.5 h, 19.2 h and 883 h to above 6769 h respectively, when the creep stresses decrease from 300 MPa, 260 MPa and 220 MPa to 180 MPa. The inter critical heat affected zone is the weakest zone of creep rupture resistance for the FSW joint of CLAM steel, the joints mainly exhibit dislocation-controlled creep deformation mechanism and the transgranular ductile fracture mode. The microstructures of inter critical heat affected zone produce recovery and subgrain boundaries are formed in here during creep process, which result in the decrease of dislocation strengthening action; the coarser M₂₃C₆ carbides is produced or the coarser Laves phase around the M₂₃C₆ carbides is formed, which result in the reduction of precipitation and solution strengthening action, these issues are the main reasons for the deterioration of the creep performance of FSW joints. The creep fracture strength of FSW joint is estimated to be 156 MPa in the condition of 1 × 10⁵ h creep life according to the Monkman-Grant equation, which reaches 88 % of the strength of base metal.