The normal value of the internal diameter of the common bile duct in our adults should be 6-8 mm, not more than 1 cm, but some elderly people may have an internal diameter of the common bile duct of 9-12 mm. dilatation of the common bile duct is defined as the internal diameter of the common bile duct greater than 1.5 cm. there are many diseases that cause dilatation of the common bile duct, the most common ones are common bile duct stones, malignant tumors of the biliary system, malignant tumors of the head of the pancreas, including patients after cholecystectomy. Their common bile ducts will compensate for dilatation. If the dilated common bile duct is found, further examination is needed to clarify the cause of the dilated common bile duct. If the cause is a stone, the stone should be removed actively, and if it is a tumor, surgery should be performed actively to cure it.