Idea of unit cell plays important roles in researches on mechanical performances and compli- cated geometric microstructure of braided composites. To 3-dimensional 4-directional rectangular com- posites braided by 4-step 1 : 1 procedure, the shortcomings of the now available methods dividing the material into unit cells are pointed out. After that,a new division methods which is right for any array pattern of the preforms is put forword. Corresponding unit cell’s graphics more conformable to the ac- tual microstructure and a set of unified formulas to determine numbers of unit cells are given too. This study is helpful to accurate master of this material’s microstructure,and provides a better basis for a- nalysis on this material’s behaviors.