This thesis gets through determination of optimum values of design variables of simple and spliced prestressed girders in bridges with considering life cycle cost of structure. For achieving above mentioned purpose, a program called OBPG has been written using fortran programming language that consists of analyze, design and optimization of structure subprograms. In this program for analyzing the structure, supposing initial values for design variables such as girders and deck dimensions and strand number, fundamentals and relations of AASHTO code have been used. Then using conclusions of structure analysis and considering design regulations, total cost of structures consisting of girders and slab concreting, prestressing cables performance, reinforcement and framework costs is minimized as function of design variables. Using optimum values of design variables that are obtained after optimization process, above mentioned stages again repeat and this repetition continues until minimum cost of project in current repetition equals with that of previous repetition with an appropriate precision. The base of life cycle optimization is different perfectly with ordinary methods of design and initial cost optimization. Although, initial cost of optimization by life cycle cost is a little more than optimization by initial cost of structure, but considering total cost of life cycle, optimization of life cycle cost is more economical than optimization of initial cost. Conclution of this thsis is a program that analysis simple and spliced prestressed girders in bridges with considering life cycle and finally gives an optimum design with considering design regulations using CONMIN optimization program and by which couses a considerable saving in total cost of project in addition to increasing precision, ease and speed of design.