In 101 jaundiced patients, gray scale ultrasonography of longitudinal scan in RAO position demonstrated the measurable extrahepatic biliary system in 73 patients : 17 cases(50%) of those with nonobstructive jaundice and 56 cases (84%) of those with obstructive jaundice. The size of the internal diameter of extrahepatic biliary system indicated that obstructive jaundice was best differentiated from non-obstructive jaundice when diameter above 7 mm served as abnormal extrahepatic duct in jaundiced patients, giving sensitivity 85.5% specificity 97% and diagnostic accuracy 89.5% by decision matrix analysis. The overall etiological diagnostic accuracy in obstructive jaundice was 40% which had higher one in choledocholithiasis and pancreas head carcinomas than other diseases. The sonography should be imaging procedure of choice in differential diagnosis of jaundiced patients, which is simple , safe, noninvasive and has high diagnostic accuracy in differentiation between the two.