Abstract: On the basis of the large unit cell of two-step rectangular three-dimensional braided composites , the finite element model of a spatial truss consisting of beam elements is developed to model the effective mechanical properties of thecomposites under the uniaxial tension. Yarns and resin are treated as beam elements. The finite element method can beemployed for predicting the mechanical properties of the composites. The uniaxial tensile tests of samples are simulated utilizing the software MARC. Results of the effective elastic properties and Poisson ratios predicted by this approach compare favorably with the experiment data. Parametric studies show that the axial elastic modulus of the composites increaseswith increasing the linear density ratio of axial yarn to braider yarns and pitch length.