Abstract: To meet the need for geometric data in finite element investigation, a study on the corner microstructure of three-dimensional(3-D) 4-directional rectangular composite preforms braided by 4-step 1∶1 procedure is made, and a accurate geometry model of the structure is developed. The vector method is used to deduce some mathematic expressions of, the axis of yarn(fiber bundle) at fabric corner, the contact curves at which a yarn at fabric corner touches with one at fabric interior & one at fabric corner, and etc. These formulas supply the geometric parameters of corner microstructure, which are useful to graphic emulation and mechanical analysis of this material.