Malignant osteoblastoma is very rare tumor which shows histological findings of an aggressive pattern and benign clinico-radiological features, and also called as aggressive or pseudomalignant osteoblstoma. The authors experienced one csse of malignant osteoblastoma involving diaphysis of left humerus in 22-year old man. The patient is healthy and maintains his normal activity without recurrence over five years after complete excision of cystic mass, curettage and bone graft.