The optimization is the process of searching for the best results in a certain load case. Complete definition of optimization includes three conditions, which are closely connected: • optimization target, defined with object function, • subject of optimization, • optimization methods. The object functions can be different process parameters such as costs, manufacturing time, costefficiency, productivity etc. In designing of the structures such as a crane there is a tendency particularly towards reducing the weight of steel structures with the same load-carrying capacity. Thus the object function can be the volume, weight or mass of the structure. In the paper we will devote ourselves to optimization of truss structures-gantry crane mean girder by means of the FEM. This method has become indispensable in all areas of analyses of structures in combination with mathematical modeling and mathematical methods of optimization and is particularly of assistance to designers in conceiving the steel structures.