Paget's disease is rare in the Chinese. We present a patient who had a rare monostotic Paget's disease involving middle diaphysis, distal metaphysis and epiphysis of left femur. Characteristic radiological findings include cortical thickening, coarse trabeculae, osseous expansion and enlargement, and bowing deformity of bone. The patient also had osteo-arthritis of the knee, which can be a complication of Paget's disease.