This research sets up a theory about style from a cognitive point of view. It has been observed that the constant applications of some factors in a design process constitute the formation of a style. Those factors included design constraints, search methods, goals, and the sequential order of applying them. A style may result from certain actions and interactions of these factors. And because of the constant applications of the factors, constant forms by which a style is manifested . The contents of the factors determine the expression of a style which can be imitated and changed from time , whereas the quantities of the factors determine the degree of a style. Thus, this theory provides, explanations about the cause, the degree, the change and the imitation of style.