Abstract: The yarn architecture of three dimensional (3 D) braided preforms produced by the four step 1×1 braiding technique has been studied by means of a control volume method in conjunction with experimental investigation. According to the movement traces of yarn carriers on the braiding machine bed, a preform was divided into three regions, i.e. interior, surface and corner, and four kinds of local unit cells were identified for it. Based on the assumption of the braiding yarns with the elliptical cross section and the introduction of the yarn packing factor, the relations between the braiding parameters were derived. Finally, the predictions of the thickness and the fiber volume fraction of preforms have shown good agreement with experimental results.