Abstract: Fracture toughness of three diffeent zones in welded API 5L-X65, namely base metal, heat affected zone and center weld, were studied in conjuction with the implementation of pipeline project for natural gas transmission in western part of Indonesia. ASTM standard method has been used for fracture mechanics tests from which the curves of load in kN vs. extension in mm have directed to the application of CTOD parameter. The results of analysis showed that the HAZ had the highest CTOD value compared with that of base metal and centre weld. Fracture surface examinations using scanning electron microscope (SEM) confirm that the HAZ has ductile fracture while the centre weld and the base metal have brittle fracture. Based on fracture toughness values obtained, precrack resistant of the pipe was analysed using leak before break criterion. It was found that the centre weld is the weakest zone. The results were used to predict wheater the pipe will break or leak during the operation of the pipe. The simulation results showed that during operation, this pipe will leak before break. Nevertheless, the pipe will break before leak when the pressure is higher than 10 Mpa. Therefore, for the operation of API 5L-X65, it is suggested that the internal pressure in the pipe should less than this critical value.