After an introduction to the procedural planning theory (PPT) which was the mainstream urban theory of the 1970's, the theoretical debate at the end of 1970's is summarized. The critiques of PPT clearly pouted out that PPT was vacuous and obsolete. An argument is made for more attention to the formalism which was the epistemological obstacle within PPT and also to the important fact the development of planning theory has always been closely related to social change.