Structuralintegrity assessments of pipelines play a key role in the design and safe operation of pipeline systems. Gas pipelines currently experience internal transmission pressures up to 15 MPa in low ambient temperatures. Combination of high strength and good toughness is essential for the steels and welded joints used in pipelines. In this study, the KIC toughness has been determined for base metal and seam weld of a pipe of grade API X65, following the ASTM E1820 standard. The API X65 steel is the most commonly used pipe material in Iran highpressure gas transportation pipelines. The fracture toughness tests employed sidegrooved and fatigue precracked compact tension specimens, extracted from the original pipe, to determine the crack growth resistance curves based upon the unloading compliance method using the single specimen technique. From these, KIC values of 302 MPam1/2 and 262 MPam1/2 were obtained for base metal and seam weld, respectively. These results produce toughness data which serve to evaluate and compare crack growth resistance of base metal and seam weld metal and to determine the critical sizes of acceptable cracks in pipelines.