Abstract: A relatively reasonable three-cell model for 3D 4-directional braided composites is proposed based on the geometrical characteristic of the yarns. The variation of bending characteristics for the yarns in a three-cell was considered in the present model. The fill factor was proposed to distinguish the different cross-sectional variations of the yarns in the three cells. The model to predict stiffness was then established based on the stiffness volume average method. The elasticity constants of the material were obtained. The effects of the processing parameters and the dimension of the structure on the effective elastic properties of the composite were also studied by the meso-mechanical method. The numerical examples of the structure with different dimensions calculated by the model show that the theoretical predictions compare well with experimental results.