This report deals with a 9-year-old neutered male Yorkshire terrier that had been suffered from vomiting, bloody diarrhea, anorexia. On exploratory laparotomy, pale-pink $10{ imes}6cm$ duodenal mass was found and submitted to the Department of Veterinary Pathology, College of Veterinary Medicine, Seoul National University. Histologically, the duodenal mass consisted of compact sheet of poorly demarcated, highly infiltrative neoplasm. The neoplastic cells were round to polygonal and contained scant to moderate amounts of granular basophilic cytoplasm and eccentrically located irregularly round nuclei with stippled chromatins. The neoplastic cells were positive to lamda light chain immunohistochemically but were uniformly negative for toluidine blue stain and giemsa stains. Based on these results, this case was diagnosed as primary extramedullary plasma cell tumor of duodenum.