In this investigation, the mechanical properties and low-temperature fracture toughness of API 5L X65 steel welded joints used for Dong Fang 1-1 gas export pipeline were studied. Structure Integrity Assessment Procedure (SINTAP)-Failure Assessment Diagram (FAD) method was applied to estimate the pipe structure with surface flaw at the weld toe. According to the ISO standard (BS7448), the CTOD fracture toughness of the welded joints were determined at the temperature of 0℃. For the heat affected zone (HAZ) specimens, post-test metallographic analysis was performed to insure that the tip of the crack was located in the coarse crystal zone in order to obtain valid and reliable results. The failure lines of analysis level 1 and 3 of weld metal were derived from the results of the mechanical property. The assessment was performed, considering the maximum lay stress, residual stress conservatively assumed to be uniform tensile stress, and minimum CTOD value. The results of the assessment showed that pipeline structure with surface flaw having the height and length respectively 2.2mm and 5mm at the weld toe is safe. This study lays the foundation of application of SINTAP to pipeline structure assessment.