Abstract: An thorough investigation on pull-out of the Φ177.8 mm casing with buttress thread connection based on analysis via relevant tests. As a result, it is identified that the pull-out problem is caused neither by the casing material nor by the over loading condition. The load imposed on the casing connection as pull-out in operation is simulated, and verification test is conducted with real casings. Comprehensive analysis is carried out based on the above mentioned activities together with the thread stabbing state before the tripping. It is concluded that the casing tripping is due to the fact that the connection threads are not made up to normal powertight position, which leads to firstly loosing of the casing connection during the flushing operation as follows due to loads upon the casing connection like vibration, and then occurrence of the pull-out failure in the course of well pressure build-up.