Osteosarcoma of the foot is rare and, when presents, commonly observed in adults. We report a case of osteosarcoma of the calcaneus in an 11-year-old girl who was presented with a 4-month history of pain and swelling on her left ankle and inability to walk in her left lower extremity. On examination, there was a diffuse swelling involving her left ankle joint and foot. The radiograph showed soft tissue with calcification and ossification that involved the calcaneus with cortical bone destruction. Histologically, the tumor was consisted of variable foci of lacy osteoid formation and spindle pleomorphic stroma with typical osteoblasts. This rare location, histologic characteristics, and differential diagnostic points are discussed.