The view of design as a kind of problem-solving activity has been an important base in the study of design cognition. Most of time, the researchers identify design problems as design briefs. But recently, some results of protocol analysis have implicated that design problems are not only about the briefs and need not exist at the beginning of design. This paper studied the roles of design problems in design thinking by protocol analysis. The results show that there are two kinds of problems

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... h occur in the design process: one is related to the brief, and the other is about designer's intention. The advanced study has been focused on the differences of the thinking mechanisms of these two kinds of problem. Many researchers have pointed to the properties that separate simple and welldefined problems from complex and ill-defined ones. Both Reitman [6] and Simon [7] discuss the nature of ill-defined problems in detail. But the criteria Simon suggests for well-structured problems are still vague. And Akin [1] considered the properties of design what constitutes the "ill-defined". When they discuss the definitions or structures of design problems, they describe the incompleteness of requirements of client, from which the ill definition of problem space is deduced. The typical point of view regards design problems as design briefs and has spurred many researchers on to study the functional problems, which are always the major topic in design briefs. These efforts focused on improvement of design methods and reasoning in dealing with complex briefs, such as the research of architecture planning. And Alexander's research (1962) is another kind. He combined the functional and formal facets of design to decompose a huge and complicated structure -an Indian village into some hierarchical small units. He tried to advance design problem solving by decomposing a big problem into some sub-problems. These researches look for to overcome the ambiguity of design problems specified in design briefs. Some characteristics of design problems in problem-solving theory can be collected as follows: 1. Design problems are regarded as the content of client briefs, which describe the goal of design, and especially as functional requirements. 2. The informational completeness is major concern for problem space definition. This incompleteness makes most of real world problems to belong to ill-structured problems. 3. The position that the problem-solving theory considers problems is for artificial intelligence, but not necessary for human designers. And most of the definitions or criteria are established for the convenient to the computational ability of a problem-solving system, which is always referred to computers but not human beings. It is a kind of prescriptive discussion on problems in the theory of problem solving.