In the present study, we described cyto-histopathological features and immunophenotyping of the large B-cell lymphoma in an 8-year-old mixed breed dog with applying the World Health Organization (WHO) system of classification of canine lymphomas. In fine-needle aspiration (FNA), lymph nodes were involved by neoplastic cells of intermediate to large size with deep blue cytoplasm; consist of centroblasts, immunoblast and medium-sized cells. Histopathologically, the follicles and sinuses of lymph nodes were replaced by sheets of numerous immunoblasts (less than 90.0% of total cells) and centroblasts. Numerous mitotic figures were also observed. Immunohistochemical analysis presented that the neoplastic cells express B-cell phenotype CD20 and CD79a, but do not stain for T phenotype CD3. On the basis of cytology, histopathology and immunohistochemical findings, the present tumor was diagnosed as diffuse large B-cell lymphoma, high-grade centroblastic type (DLBCL-CB) according to WHO histological classification. Applying this classification system for diagnosis of canine lymphomas is very useful and has a high accuracy and consistency. However, further co-operative studies between clinicians and pathologists should be performed, in order to improve the effectiveness of this classification.