Introduction– Osteosarcoma are the most common primary malignant bone tumour exclusive of multiple myeloma, principally arises in metaphysis of long bones of children and adolescent. Rarely, tumour may arise in flat bone, and rib involvement is very infrequent. Case report– we reported a rare case of primary chondroblastic osteosarcoma in 35 year old female present with rapidly growing mass over chest wall without associated history of any trauma. Radiological imaging suggest diagnosis of chondroblastic osteosarcoma, subsequent HPE correlation confirmed the same. Conclusion– Osteosarcoma though common tumour arising from long bones, can arise from flat bone like ribs. Main modality of treatment is surgical excision followed by adjuvant systemic chemotherapy with high cure rates.