Infantile fibrosarcoma is a rare soft tissue malignant tumor, when it occurs, it is usually seen in the first year of life. The clinical course of infantile fibrosarcoma is more favorable and metastasis is rare compared with that in adulthood. While adult fibrosarcoma are common in the thigh, infantile fibrosarcoma affect chiefly the distal portions of the extremities. Standard treatment is primarily wide surgical excision. In this case report, we present our experience of an infantile fibrosarcoma of thigh with good clinical course 36 months after tumor resection and the usefulness of detecting the ETV6-NTRK3 gene fusion in differential diagnosis.