Stress fractures of the olecranon caused by repetitive stress force have infrequently been reported as a cause of elbow pain in adult athletes, engaged in throwing and pitching sports. We diagnosed as a stress fracture of the olecranon by clinical and radiographic findings and treated surgically. The patient returned to playing baseball at a competitive level and was asymptomatic 4 months after the first operation. However, the patient re-injured the olecranon and a second surgical treatment was performed almost 1 year after the first operation. After the second surgery, the patient returned to playing baseball at a competitive level and was free from elbow symptoms. We presented a stress fracture of the olecranon in a semi-professional adult baseball player and suggested that surgical treatment is necessary.