Thermo-mechanical control processing is used to produce API pipeline steels. To design a proper thermo-mechanical cycle, it is needed to determine the critical temperatures including non-recrystallization temperature (Tnr) and austenite to ferrite transformation start and finish temperatures (Ar3 and Ar1). In this research, average schedule and continuous cooling torsion after a real schedule were used to determine critical temperatures of API X65 steel for the first time in Iran. This steel is imported from abroad and is extensively used in Iran for large diameter, high-pressure gas transportation pipelines and for oil transmission networks. It was found that the average schedule was a proper method to determine Tnr; while, continuous cooling torsion was proper to determine Ar3 and Ar1. The obtained results were compared with Boratto and Ouchi experimental relations with the purpose of evaluating the reliability of these relations for determination of Tnr and Ar3 critical temperatures. The obtained 4 percent relative error from both relations showed the need of conducting the experimental studies. With regard to the lack of experimental data, the obtained results can be used to design the optimum thermo-mechanical control process through the selection of proper temperature ranges for rough and finish rolling stages in domestic manufacturing of the test steel.