An 8-year-old male mixed dog was referred to the National Chiayi University Veterinary Hospital with a 10-day history of panting, vomiting and anorexia. Blood and serum examinations revealed anemia, dramatically leukocytosis (59, 200/μL), and slightly increased activity of alkaline phosphatase (ALKP), alanine aminotransferase (ALT), asparatate aminotransferase (AST) and total bilirubin (T-Bili). Thoracic radiograph showed a radiopaque image appeared around the hilus. Lymphoma was suspected through biopsy examination. At postmortem examination, numerous masses along the mediastinum, and the enlargement of liver and gallbladder were noted. Histopathologically, round or oval tumor cells with high nucleus-to-cytoplasm ratio and variable size and morphology of nuclei were observed in the tumor tissue, and the infiltration of tumor cells into lung tissue was also observed. Fibrosis of alveolar septa and fibrinous accumulation within the alveolar lumens were also noted at the histological section of the lung. Diffuse, acute swelling and vacuolar change of the hepatocytes, and the accumulation of bile pigments and the hyperplasia of bile duct were also observed. The majority of tumor cells expressed B-cell phenotype as demonstrated by its immunoreactivity with anitcanine CD20 antibody. Based on the histopathological characteristics of the thoracic tumor, mediastinal B cell lymphoma is diagnosed in this dog.