The pancreatic endocrine tumors are uncommon neoplasms and are classified into non-functional and functional tumors. According to whether the secreted hormones are originated from pancreatic islet cells or not, the tumors are also classified into normotopic and ectopic tumors. Except for insulinoma, more than 60% of them reveal malignant behavior. The presence of endocrine tumor is diagnosed when patients develop a hormone-specific symptom, and the location of tumors are usually diagnosed by a combination of ultrasonography, computed tomography, magnetic resonance imaging and selective angiography. A somatostatin receptor scintigraphy is promising. Nevertheless, these examinations occasionally failed to precisely locate the tumors especially when they are very small and/or multiple. For such cases, both portal venous sampling and arterial provocation test are helpful.