Abstract: The relation of J-value to other parameters and torque wrench moment of API round threaded connection is analysed based on the detection to thread parameters of \API73.0×5.51mm EUN80 tubing connection. The conclusion that J-value changes with the change of thread parameters in optimum torque wrench moment. Controlling slack level of threaded connection by controlling J-value cannot ensure threaded connection is in optimum state. Therefore, it is unscientific that for user to judge slack level of threaded connection of tubing or casing by J-value.