Long span mobile bridges (LSMBs) are widely used in military maneuver operations or civil rescue works across narrow gorges or rivers. Especially it is quite a difficult problem to design LSMBs of over 50 m length satisfying required safety criteria under military load class 70 (MLC 70), namely permit vehicle weight of 63.5 tons. In order to optimize a LSMB of 60 m length for both lightweight and minimum deflection under MLC 70, a multi-objective optimization process based on genetic algorithm (GA) and weighted sum method for handling multiple objectives was utilized after its validity verification with the six-member and five-node truss problem. GA in this study utilized a variable penalty function to treat design constraints and fitness function to deal with both objective and penalty functions together. Finite element structural analysis of the LSMB was done by ANSYS for the design optimization process. As the result, the optimally designed LSMB of 60 m span that meet requirements and safety criteria of a military bridging equipment design and test code has been successfully obtained.