In this paper, optimal design of the second arm in a SCARA robot was studied. The mass and moment of inertia of the second arm of a SCARA robot have great effects on performance indices such as cycle times and torques of the first and second axes. To reduce the mass and moment of inertia, optimal design was carried out by FEM analysis using parameters such as width and height of the arm rib, which was newly adopted to decrease the arm thickness in keeping stiffness. Computer simulation was conducted in X and Y directional paths. As a result of the optimal design, maximum torques of the first and second axes decreased by 10.1% in maximum.