In this study, micro genetic algorithm is used to find an optimum cross section of grid structures with pretension. Design optimization of trusses consists of arriving at optimum sizes of cross-section and prestressing force parameters, when weight of the truss is minimum, satisfying a set of specified constraints. The present approach is verified by ten-bar truss example showing good agreements with previous results. Features of the proposed method, which help in modeling and application to optimal design of pretensioned truss structures, are demonstrated by solving a problem of seventy two bar truss structures. The minimum weight design of seventy two bar truss is performed for various magnitudes of pretension and optimal prestressing forces are also found for various configurations of pretensioned truss structures.