Abstract: The fracture behavior of underwater wet friction taper plug welded joints for X52 pipeline steel was investigated with crack tip opening displacement（CTOD） test. Effects of post-weld heat treatment on fracture toughness and microstructure characteristics of welded joints were discussed. The results showed that the average CTOD value of weld center was only 0. 103 mm with significantly lower fracture toughness than that of parent metal,due to the obviously heterogeneous quenched martensite and a small amount of widmanstätten in weld center. With the tempered temperature increasing from 500 ℃ to 800 ℃,microstructure of weld center transformed into tempered martensite and become more homogeneous,which effectively improved the fracture resistance of weld center. The average CTOD value of weld center after 800 ℃ tempered temperature reached over 0. 8 mm. However,after tempered with suitable temperatures,bonding zone could only get a limited improvement in fracture toughness,because of inclusions in bonding zone. The maximum CTOD value of bonding zone was 0. 3 mm after 800 ℃ tempered temperature. The fracture resistance of bonding zone was a vital factor which affected the application of underwater wet friction taper plug welding technique.