Abstract: A unit cell geometrical model for 4-step 3D 4-directional rectangular braided composites is proposed. The effect of bending and cross-sectional variation of the yarns due to their mutual squeeze is considered in the model. A model topredict stiffness is then established based on the stiffness volume average method and on the laminated plate theory by assuming that differential yarns are straight . Thus the elastic material constants can be obtained. Numerical results show theeffectiveness of the model. The influences of the technics parameters and the cross-sectional shape of yarn on the engineering elastic constants are also studied , and some conclusions are drawn based on the results reported herein.