Abstract: The space grid of fiber microstructure of 3-D multidirectional braided composites provides more tailor flexibility in respect of the performance of the composites and their structures.To utilize the tailor-ability,genetic algorithm is adopted to optimize the topology parameters of fiber microstructure,linear density and pitch length so that the 3-D braided composites satisfy the lower limit requirement of stretch modulus and the minimum ratio of material mass to lateral Poisson.Research results show that on the base of the geometric equivalents between the space truss unit cell and the composites microstructure,the optimal topological microstructure can be found for the elastic demands of the composites by the methodology in which the genetic algorithm is combined with the finite element stress analysis of the space truss model of unit cell.