Abstract: The suppositions of the cross-section shape of yarns, especially weft yarns, in composite materials are different when predicting mechanical properties of three dimensional woven reinforced composite materials. The roundness, raceway, convex and rectangle shapes were assumed as the cross-section of weft yarns while the rectangle shape was assumed as the cross-section of warp yarns. In this paper, the elastic modulus was predicted for four different angle-interlock structural composite materials using the stiffness averaging method and the iso-strain mechanical model. Contrasting with digital photos of the composite materials and with experimental results of the elastic modulus, the result shows that the roundness assumption of weft yarn cross-sections is far inaccurate and the predicting result of elastic modulus under the condition of the roundness assumption is much smaller. There is little difference in the predicting results of elastic modulus between raceway, convex and rectangle cross-sections of the weft yarn assumptions and the predicting results are in better agreement with the experiment results of the elastic modulus.