Abstract: This article reported three patients diagnosed as insulinoma with typical Whipple triad. Many examinations such as CT, US and selective arteriography failed to indentify the site of the lesion before operation. A method of rapid assay of blood insulin from portosplenic vein during operation was found to be helpful for localization of it. The result of using this method made clear that: 1. The tumor of pancreas in two pitients were insulinomas as the site of peak value of insulin is near the Site of tumors palpated during operation. 2. There was insulinoma in the body and tail of pancreas in other patient because of the result that there was no high value of insulin in his splenic vein. This method to take blood from portosplenic vein is different from the routine method. The method of catheterization and its significance were also discussed in details.