Background: The treatment of diaphyseal forearm fractures using open reduction and plate fixation is generally accepted as the best choice in many studies. However, periosteal stripping, haematoma evacuation may result in delayed union, nonunion and infection. Refracture after plate removal is another concern. To overcome these problems intramedullary nails (IM) with different designs have been used with various outcomes. However previous IM nails have some shortcomings such is rotational instability and interlocking difficulties. We evaluated the results of newly designed IM nail in the treatment of diaphyseal forearm fractures in adults. Materials and Methods: 32 patients who had been treated with the interlocking IM nail for forearm fractures between 2011 and 2014 were included in this study. There were 23 males and 9 females with mean age of 36 years (range 18-68 years). 22 patients (68.8%) had both bone fractures. Nine patients (28.1%) had open fractures. The remaining ten patients (31.2%) had radius or ulna fractures. Grace and Eversmann rating system was used to assess functional evaluation. Patient reported outcomes were assessed using the Disabilities of the Arm, Shoulder and Hand (DASH) questionnaire scores. Results: Union was achieved in all patients. The mean followup was 17 months (range 13 - 28 months). According to the Grace-Eversmann criteria, 27 patients (87.5%) had excellent or good results. The mean DASH score was 14 (range 5-36). Overall complication rate was 12.5%. Superficial infection was encountered in two patients. One patient had delayed union, however fracture healed without any additional surgical procedure. One patient who had open grade 3A, comminuted proximal third radius fracture developed radioulnar synostosis. Conclusions: The new design IM interlocking forearm nail provides satisfactory functional and radiological outcomes in the treatment of adult diaphyseal forearm fractures.