The aim of this research is to set up a theory about style in architectural design from a cognitive point of view. It has been observed that the constant application of certain factors in a design process constitutes the formation of a style. Those factors include design constraints, search methods, goals, and the sequential order of applying them. Because of the constant application of these factors, constant cognitive phenomena appear and, consequently, produce constant forms by which a style is manifested. Thus, a style results from the operations of these factors and the interactions among them. The contents of the factors determine the expression of a style, which can be imitated and changed over time, whereas the quantities of the factors determine the degree of a style. Therefore, this theory provides explanations about the cause, the degree, the change, and the imitation of a style in design.