Primary cardiac sarcomas are very rare. We report one case of primary cardiac osteosarcoma arising from the left atrium of a 42-year-old woman and analyze its clinical and pathological features. Histopathologic examination revealed the tumor was composed of massive osteoid and cartilaginous differentiation embedded in spindle and polygonal cells with marked cytological and nuclear pleomorphism. The patient was alive without recurrence or metastasis at 20 months after surgery without further adjuvant chemotherapy or radiotherapy.