This paper presents the method to solve reliability based design optimization (RBDO) of steel box girder bridge using computer program which is developed by integrating Matlab optimization toolbox and reliability analysis subroutine. Moreover, the effect of environmental agents is considered in terms of modeling the uniform corrosion phenomena which reduced the ultimate and service capacity in the structural lifetime. All reliability constraints are formulated in term of ultimate and service limit state functions according to AASHTO code. The general procedure of optimization analysis is proposed and applied to the simply support steel box girder bridge with reinforced concrete slab. In the optimization process, the steel box girder is represented by a number of deterministic design variables including the depth of girder, the thickness of web, top flange, and bottom flange. Sensitivity analysis is performed for investigation the effect of span length and target reliability index level on the optimum solution.