The purpose of this research is to study the cognitive process in architectural design problem solving. It also will explore a cognitive structure (model) capable of representing the problem solver's cognitive behavior. The goal plan, schemata, perceptual-test, and generate-and-test are regarded as cognitive mechanisms that evolved in the problem solving process. They were observed in an experiment in which an experienced architectural designer was asked to do a residential design.Results from protocol analysis showed that an invariant cognitive structure could be built upon these cognitive mechanisms to explain the problem solving behavior. This cognitive structure (model) also provides a framework for future simulation.