This paper proposes a method to find the preferable workspace for fatigue life improvement of robots with flexible joints under percussive riveting. The development is motivated by the growing interest in using industrial robots to replace human operators for percussive riveting operations in aerospace assembly. A most important characteristic of robotic percussive riveting is the repetitive impacts generated by the percussive rivet gun. These impacts induce forced vibrations to the robot, and the joint shaft fatigue due to the resulting stress cycles must be prevented. This paper aims at finding the preferable workspace for fatigue life improvement of the robot, that is, the end-effector positions where the joint stresses are below the endurance limit. For this purpose, a structural dynamic model is established for the robot under percussive riveting. Then, an approximate analytical solution is formulated for the torsional stresses of the robot joints. Once the distributions of the stresses are obtained over the workspace, the preferable workspace for fatigue life improvement can be found by comparing the stresses with the endurance limit. Simulation studies are carried out for a mobile robot under percussive riveting. It is found that the dynamic response of the robot to the percussive riveting varies dramatically over the workspace. The method is then used to obtain the preferable positions of the robot end-effector for fatigue resistance.