Abstract: The joint of locking screw with nylon insert is studied and the locking mechanism is analysed through experiment and an example of application. Then the calculation equation for tightening torque is deduced based on the experiment results. At last, empirical equations for the structure design of locking screw with nylon insert are presented and an example of application on air plane equipment is also given. The structure of a screw with nylon insert is that nylon is formed into cylinder or strip which is inserted into a hole in the thread portion of the screw. When the screw is tightened, the nylon in the screw is pressed and makes a resistant torque, when loosened, the screw must overcome this torque and thus is locked. The resistant torque prevents the loosening of a screw when the preload of the joint decreases or disappears.