Abstract: The crystal structures of R_1+εFe₄B₄ compounds which were found in the permanent magnets R-Fc-B systems arc all built based on the two interpenetrating substructures. Fe-B substructure and R substructure, both have tetragonalsymmetry. Since the two substructures do not have the same periodicities in c direction, R_1+εFe₄B₄ compounds form the so called Chimney-Ladder structureor Vernier structure which may belong to the incommensurate composite crystal structure. The translation symmetry of the incommensurate structure do not exist in three dimensional space and the superspace group theory has been developed for the description of the symmetry of incommensurate structure. In this paper, the superspace group theory is applied to the R_1+εFe₄B₄ compounds. The result shows that the symmetry of R_1+εFe₄B₄ compounds can be descripted by the general form P_S1S1^P4/nmm