Agenesis of dorsal pancreas is a very rare congenital anomaly which comes from the failure of development of the body and tail of pancreas in embryogenesis. Castleman`s disease is a rare lymphoid tumor which usually occurred in the mediastinum, but it is very uncommon in the retroperitoneal pancreatic tail area. We report a case of partial agenesis of dorsal pancreas with Castleman`s disease in pancreatic tail area, mimicking a pancreatic tumor. A 46-year-old woman was admitted to our hospital with intermittent abdominal discomfort for 2 months. Computed tomography, magnetic resonance imaging, and endoscopic retrograde cholangiopancreatography (ERCP) revealed a tumor at distal pancreatic tail area, short pancreatic duct, and absence of pancreatic body and tail. The surgical excision of the tumor revealed hyaline-vascular type Castleman`s disease.