Abstract Background: The human biliary system consists of the liver that synthesizes bile and the intra hepatic and extra hepatic biliary passages. The extra hepatic biliary passages include the gall bladder that stores and concentrates the bile and the right and left hepatic ducts that unite to form the common hepatic duct that unites with the cystic duct to form the common bile duct (CBD). The common bile duct descends in the right free margin of the lesser omentum then behind the first part of the duode-num and the head of pancreas then joins the main duct of pancreas forming the ampulla of Vater (Hepatopancreatic ampulla) that opens into the posteromedial aspect of the second part of duodenum. Aim of Study: This study was done for assessment of the mean and upper limit diameter of the CBD in the different age groups between 20 and 80 years and after cholecystectomy more than 2 years agofor early prediction of the obstruction or narrowing of the lower part of the CBD. Subjects and Methods: This study was done on 1000 patients came to Sohag University Hospital with problems not related to the biliary system and examined by magnetic resonance imaging (MRI), magnetic resonance cholangio-pancreatography (MRCP) and trans abdominal ultrasonogra-phy. Results: The study demonstrated that, the diameter of CBD increases gradually with age with more widening of CBD after cholecystectomy for more than two years and with no sex related differences in the different age groups. Conclusion: The diameter of the non-obstructed CBD is proportionally related to the age and increases after cholecys-tectomy with an upper limit for each age group.