Abstract: To realize the tailor ability of 3-D braided composites for applications,the microstructure parameters of 3-D braided composites are optimized to meet the need of macro stiffness property based on the micromechanics methodology.The truss model of unit cell is developed based on non-continuum mechanics,and finite element method is utilized to predict the effective elastic modulus of composites.With weight minimum and stiffness requirements as the objective,the genetic algorithm is used in the optimization of microstructure parameters of the composites.The comparisons between the computational and experimental results show that the combination of the truss model of unit cell and genetic algorithm can easily find out optimal microstructure parameters to satisfy the elastic requirements of 3-D braided composites.Here the microstructure parameters include the yarn linear density and the pitch length.