A case of osteosarcoma arising in the patella was reported. The patient was a 51-year-old male, complaining of pain and swelling in the patellar region for 5 months duration. On physical examination, a large tumor measuring 15×7cm was palpated in the patellar region. All laboratory studies were within normal limits. Rentgenograms revealed a geographic destruction of the distal half of the patella. A thin periosteal shell was found in the anterior aspect of the tumor, but no demineralization was noted within the tumor. A CT scan demonstrated expansion of the patella and extensive infiltration of the tumor to the soft tissue. Angiograms demonstrated a slight increase in number of small arteries to the tumor. An above-knee amputation was performed followed by an open biopsy. Histologically, the tumor was composed of tumor cells with slight atypism, benign giant cells and woven bones. A diagnosis of well to moderately differentiated osteosarocoma was made. No anticancer agent was administered. The patient is disease free 2 years and 2 months after surgery.