The problem of robust stability of interval systems under multiplicative H∞ perturbations is studied. It is proved that the stability of interval systems under multiplicative H∞ perturbations can be guaranteed by the corresponding properties of 12 key vertex systems. The small gain theorem of robust form is obtained. The problem of robust stability of interval systems under multiplicative H∞ perturbations is studied. It is proved that the stability of interval systems under multiplicative H∞ perturbations can be guaranteed by the corresponding properties of 12 key vertex systems. The small gain theorem of robust form is obtained.