The fractures of the forearm bone are common and the forearm has the specific movement of supination and pronation. So, the goal of the treatment of forearm fractures is the recovery of rotatory function of the forearm as well as the function of the elbow and wrist. Surgical treatment usually is not necessory in children under 10 years of age because of remodelling potential and spontaneous correction ability. But, anatomical reduction and rigid fixation is essential in fractures of adult forearm above 15 years of age because of rotational deformity and angulation after forearm fractures may result in serious functional problems of the forearm. The purpose of the our retrospective study is to compare the clinical result between the plate fixation and intramedullary nailing of the diaphyseal both forearm bone fractures in adult. We reviewed 64 patients above 15 ages who had diaphyseal both forearm bone fractures, and were treated with fixation using compression plate or Rush pin. Forty patients were treated with both plates, 7 patients with both Rush pins, and 17 patients with plate and rush pin. Galeazzi or Monteggia fractures were excluded in this study. On final follow up, we performed the radiological analysis and compared the operation interval, immobilization period after operation, bone union time, functional result and complications in these groups. Functional results was more higher in both plate fixation, and complications were high in both intramedullary nailing. In conclusion, both plate fixation is the best treatment method in the diaphyseal both forearm bone fractures in adult. Thus in both forearm bone fractures, both plate fixation is recommended, but if it is not available, at least one bone with plate fixation is necessary