An additional inner-feedback loop is used to reduce the channel interaction of a multivariable control system such that the design of outer-feedback loop can be treated as a single-variable problem for ease in achieving the required main-channel performance. The relative error between the exact input-output relation t(ii)(j omega> of channel i and its approximation (t) over cap(ii)(j omega> = {l(i)(j omega)f(ii)(j omega)}/{1 + l(i)(j omega)} is introduced to provide an effective measure for assessing the achieved noninteraction. According to the approach proposed, the single-input/single-output quantitative-feedback-theory design method can be applied directly for the design of both inner- and outer-feedback loops.