The contribution deals with the problematics of topology optimization of the arm of a mobile robot manipulator for the international competition European Rover Challenge (ERC). The manipulator was designed at the Department of Robotics at VSB - TU Ostrava and is described in detail below. The aim was to test the entire design process: determination of loading forces, material model and parameters identification, maximal allowed stress in the structure, the topology optimization, and the manufacturing process of the arm by FDM additive technology. The process has been tested for his future automation. The aim was also to create the new arm lighter than the original one, but the reliability must be satisfied.