We report large tumors of the left thoracic cavity, which arose from the ribs, were diagnosed as chondrosarcoma in a 4-year-old male mixed dog. The dog was presented with swelling in the left side of the chest wall and lameness. The masses were found to be circumscribed, whitish-grey, large and firm. The cut surface revealed whitish-grey lobules of varying size with cartilaginous consistency. Those were subsequently metastasized to the lung and mandibular lymph node. Histologically, the tumors were made up of clusters of chondroid cells having pleomorphic hyperchromatic nuclei with occasional mitosis. A special and immunohistochemical staining was performed to confirm the diagnosis. Chondroid matrix in tumor showed a positive reaction for alcian blue-periodic acid-Schiff staining. Immunoreactivity to S-100 proteins was present in the cytoplasm and/or nucleus of chondrocytes in tumor. The final diagnosis was grade III chondrosarcoma in the rib, considering histological features in grading criteria.