Abstract: Annular internal overlap, which is different from regular helical internal overlap, was generated in the production of φ177.8 mm×9.19 mm BG100 casing pipe, and led to seriously influence to the product quality. By means of macroscopic and microscopic examinatin, the cause of annular internal overlap was analyzed. The results showed that the oxide slag and its secondary oxide which nearly circumferential distributed in the billet core were the main reason which caused the annular internal overlap.