Based on the composite compounds Ho2Ni7-xFex (x=0-3.0), a method of describing the structure transition from rhombohedral to hexagonal is discussed in terms of free electron concentration. The transition is investigated by X-ray powder diffraction and magnetic analysis. The compounds crystallize into the rhombohedral Gd2Co7-type structure for x=0-0.5 and into the hexagonal Ce2Ni7-type structure for x=0.5-2.5. The values of lattice parameters a and c of the Ho2Ni7-xFex compounds increase with the addition of Fe, and the saturation magnetization Ms decreases with Fe content increasing at a rate of d Ms/d x=-2, manifesting antiparallel alignments of the Fe and Ho moment. The higher the free electron concentration, the stabler the rhombohedral structure is, otherwise the hexagonal structure is stabler, which provides a meaningful parameter to distinguish the two allotropies in composite structure intermetallics.