Unstable fractures of the proximal humerus should be treated with precise reduction and stable fixation, and early joint motion should be permitted. But stable fixation of the proximal humerus is frequently difficult to obtain in older patients due to osteoporosis and fracture comminution. We treated one case of a segmental comminuted fracture of the proximal humerus with severe osteoporosis with a method of inlay fibular autograft and fixation with a helical locking compression plate (LCP). Stable fixation was obtained, so early motion of the shoulder joint was permitted. The fracture was healed in 12 weeks after the operation without loss of fixation and there were no problems at the donor site of the fibula. Functional recovery of the shoulder was satisfactory. The result of Neer's functional score was 87 points (satisfactory) and Constant score was 83 points.