Rupture of the triceps tendon is a rare condition. We report a case of triceps tendon rupture with avulsion fracture of the olecranon with discussion about pathogenesis from the viewpoint of healing of the olecranon ossification nucleus. An 18-year-old man presented with avulsion fracture of the olecranon and triceps tendon rupture. Operative treatment was conducted with good results. Rupture of the triceps tendon is a rare injury comprising approximately 2% of all tendon injuries, and avulsion at insertion into the olecranon is the most common site of rupture. This injury is most likely to happen in young people as sports-related trauma. Among young people who have insufficient healing of the olecranon ossification nucleus, avulsion fracture of the olecranon may occur due to strong contraction force of the triceps tendon. Our clinical evidence may suggest that avulsion fracture of the olecranon is related in olecranon ossification center healing. Most triceps tendon ruptures are accompanied by avulsion fracture of the olecranon, and it is important to suspect this injury when radiographs show a small fleck of bone avulsed from the olecranon.