Two cases of choledochal cyst are presented which the correct diagnosis was obtained preoperatively by ultrasound. Finding on sonography included a large cystic mass in porta hepatis separating from gallbladder; The dilatation of intrahepatic duct is noted on one case. Intravenous cholangiography was perfomed in one case and a perfect visualization of choledochal cyst was obtained. However, the accurate preoperative diagnosis of choledochal cyst made by sonography.