One 203mm drill collar failed infrequently just after 200h of pure drilling time. This paper gives a systematic analysis in consideration of both material quality and loading condition on drill strings. Measurement and inspection were performed on configuration, chemical composition, mechanical performance, metallograghy and micro-fractograghy. It’s thought that the drill collar failure accident was premature fatigue failure accident. Failure reason is the mechanical damage caused by improper manufacture process in the root of thread. Under the effect of fluctuating stress, fatigue cracks originated from mechanical damage position and grew rapidly. When cracks propagated through the wall thickness, drill collar fractured.