Abstract: There is a difficult problem for composites designers to solve; that is, how to confirm the relationship between preform structural design parameters and composites thickness.Based on observing and analyzing digital images and microstructure of 3D woven composites cross-sections,the results show that the position of deciding the thickness of 3D woven composites is located at the cross-section which passes through the center of the weft thread,the cross-section of a warp thread is rectangular,and the longitudinal track of a weft thread is linear.Furthermore,thickness equations for 3D woven composites are deduced,which are expressed by preform structural design param-eters.In the end, an example is given to certify the validity of the thickness equations.The result shows that the thickness equations are correct and practical.