This study explores the effect of exercise-induced fatigue on upper extremity stability. The kinematic changes of the upper extremity during bench-press exercise are quantified by measuring the flexion angle and range of motion of the elbow joint， the amplitude of the pitch angle， the amplitude of the yaw angle， and the perturbation frequencies of the pitch and yaw angles， respectively. Fifteen healthy males performed bench presses until fatigue set in. The joint kinematics in the initial stage and the fatigue stage of the bench-press exercise are calculated and analyzed using a laboratory-developed three-dimensional motion analysis system. The flexion angle and range of motion of the elbow joint are significantly higher in the initial stage of the bench-press exercise than in the fatigue stage. The amplitude of the pitch angle in the fatigue stage (mean 11.98°, standard deviation (SD) 3.52°) is significantly higher than that in the initial stage (mean 2.85°， SD 1.13°). The amplitude of the yaw angle in the fatigue stage (mean 8.87°， SD 2.05°) is significantly higher than that in the initial stage (mean 3.27°, SD 1.70°). Finally, the perturbation frequencies of the pitch and yaw angles, respectively, are both significantly higher in the fatigue stage than in the initial stage. The results provide useful insight into the relationship between fatigue-induced perturbation frequency and the instability of the upper extremity during bench-press exercise. An unstable upper extremity posture may increase the risk of injury during the fatigue stage of repetitive exercise.