Abstract: The accident of ?88.9 mm square kelly break in well WS1 caused great economic losses, and a study was conducted to probe the causes of this accident. Macro analyzing method, as well as micro method of using scanning electron microscope was applied to identify and analyze the kelly break-fracture, and the features of fatigue failure were identified. Test analysis was also carried out to determine the chemical composition, mechanical property and metallographic structure of the kelly, and it was concluded that the material strength and toughness is not sufficient enough. Study and test results showed that the break took place at the structure mutation location, and it was of brittle failure due to earlier fatigue fractures, and the main causes of kelly break were unqualified raw material and poor machining quality. This study can be used as a reference to prevent the happening of such accidents in the future.