OBJECTIVE To analyze the causes of delayed union or nonunion of the ulna after intramedullary nailing in pediatric forearm fractures. METHODS From February 2005 to February 2010,5 patients with forearm fractures who were treated with titanium elastic nailing (TEN) were identified to fulfill the criteria of having developed a delayed union or nonunion of the ulna. The causes of delayed union or nonunion were investigated according to mechanism of injury, fracture location, treatments methods and postoperative management. All patients were male and the age was 3 to 14 years old with an average of 9.4 years. All fractures were located on the mid-third part of forearm. Two cases had a re-fracture. Among them, 3 cases caused by high-energy injury and 2 cases by falling down. Open reduction were performed in 4 cases while the other one was treated with closed reduction. Four patients were immobilized in an above-elbow cast, postoperatively. RESULTS All patients were followed up from 7 to 19 months with an average of 11.4 months. There were 4 delayed union and 1 nonunion. Three patients healed after the removal of the nail and avoidance of weight-bearing. Two patients healed by replacing another fixation. No patients had soft-tissue irritation or nail-entry-site infections.. The clinical effect was evaluated according to Daruwalla and Price scores with 3 excellent and 2 good of the results. CONCLUSIONS Using titanium elastic nailing for the treatment of pediatric both-bone forearm fractures is a good method. However,strict indication selection should be followed to avoid delayed union or nonunion.