  Using the vertex operator approach we show that fusion of the RSOS models can
be considered as a kind of coset construction which is very similar to the
coset construction of minimal models in conformal field theory. We reproduce
the excitation spectrum and $S$-matrix of the fusion RSOS models in the regime
III and show that their correlation functions and form factors can be expressed
in terms of those of the ordinary (ABF) RSOS models.
