Gantry cranes are particularly suited to lifting very heavy objects, especially in shipbuilding industry are used massive objects like ships’ engines to be lifted and moved over the ship. Capacity of the overhead gantry cranes increased proportionally through the needs. In this study dual-trolley heavy duty overhead gantry crane that can carry loads up to 800 tons was designed and analyzed. Stress and displacement values of main beam were calculated by both Finite Element Method and using mechanical formulas. As a result, it has been seen that, F.E.M. is a quite practical method to handle stress analysis problem of gantry cranes.