Abstract: Analyzed here in the paper are the apparent shape,the SEM morphology,and the macro structure and micro structure of the fracture of the Φ73.02 mm×5.51 mm N80 non-upset tubing.Compared are the chemical compositions,the mechanical properties and the metallographic test results of the coupled pipe body with those of the fractured pipe body.And normal thread make-up status of the tubing is described.The test result leads to such a conclusion that the physiochemical properties of the material of the failed tubing is in compliance with the requirements as specified in Q/SY 1572.2-2013,and the difference between the structures has nothing to do with the fracture;and that there is some fatigue caused fracture near the cracking source that is gradually expanding inward from the surface,and getting bigger and bigger,and finally causes the breaking of the tubing as a whole.In addition,other elements affecting the on-the-site thread fracture are make-up of the coupling with over torque,tilting of the oil string,and the offset stress borne by the tubing.Therefore,it is suggested that sample analysis of the well liquid be taken to further identify the causes for the said fracture.