Function is a key concept to integrate object modeling and process modeling in design. In this paper, function is defined as a part of FBS (Function-Behavior-Structure) diagram, where function is a description of behavior abstracted through recognition of behavior for utilization. Function defined above is then used in FEP (Functional Evolution Process) to represent design processes. In FEP, function can be evolved in four ways, i.e., decomposing evolution, causal evolution, "patch" evolution...