Abstract: A mode for classifying program transformations, and a method for formally expressing program transformations by 4 tuple are proposed.Transformation in the second type by the classifying mode is more general, during which process not only the object but also the rule base and the knowledge base are modified.A specification for the second type of transformations is illustrated and the Specification Metalanguage of Program Transformations, SMOPT, is presented. The specification and the metalanguage have been applied in the design and the implementation of AB transformer of the decompilation system DECLER.