An unusual case of malignant pancreatic composite tumor with both components of acinar cell tumor (ACT) and islet cell tumor (ICT) was investigated histologically, immunohistochemically, and ultrastructurally. The pancreatic tumor with central cyst formation was found on computerized tomographic examination of a 72-year-old man reporting appetite and weight loss. The ACT component was present in the original pancreatic region and the ICT region was adjacent to the ACT. ACT was immunohistochemically positive for pancreatic amylase, whereas ICT had argyrophil tumor cells immunohistochemically positive for chromogranin A. There were several tumor cell nests positive for both pancreatic amylase (acinar differentiation) and chromogranin A (islet differentiation). We speculated that ICT may have arisen from the de-differentiated tumor cells in the ACT after the occurrence of ACT.