Abstract: Analyzed in the paper are the causes for mill-end tropout failure of the casing-used long round thread connection.The analysis involves such activities as observing the macro-morphology of the failure sample,testing the physical and chemical properties of the casing material,conducting a contrastive analisis of the dropout coupling's OD at different positions,checking the thread tamper of the failure sample,performing contrative physical obeject tests to samples from casings of the same lot,and making FE simulation analysis of the thread connection under tensile conditions,etc.The result shows that the yield strength of the failure casing's material is less than the value as specified under API Spec 5CT-2011;the make-up position of the thread connection mill-end is up to the requirement under the applicable specification;the dropout couple's outside diameter at different axial location is consistent and shows little difference;and variation scope of dropout couple's thread taper at mill-end is rather high.Moreover,it is inversed via the FE simulation analysis that it is imposible for the thread connection mill-end to drop out at the make-up trace location.It is therefore concluded that the said dropout failure is caused mainly due to the rather low yield strength of the casing material,and looseness of the thread connection itself as caused by the on-the-site float make-up operation.