This paper discusses the transformation ability of Burstall and Darlington s folding/unfolding system, i.e. what kind of programs can be derived from a given one. It is a generalization of the problems of correctness and completeness. Tn the paper, we proved a necessary condition of transformability, and got a bound of efficiency improvable by transformation. The partial correctness and incompleteness of the system are corollaries of the condition.