Abstract: Functional size measurement (FSM) method is one of the important researching fields in software measurement and the organization of measurement usually measures the software size by using different FSM methods to satisfy different users' needs. In order to measure software by simultaneous using more than one FSM method, this paper analyzes three FSM methods (IFPUG FPA, COSMIC FFP and MarkII FPA), proposes a unified model of functional size measurement (FSMUM), and gives the mapping rules from FSMUM to three FSM methods. Finally, a case study using FSMUM is provided.