Scalability is an important performance criterion of parallel computing. However the conventional scalability metrics are not suitable for SMP(symmetric multiprocessor) cluster. How to measure scalability of SMP cluster? This paper proposes a solution to the problem. It first finds out and verifies the reason, nonequivalence of the processor sets and then it adopts the viewpoint of processor set to observe the behaviors of the system correctly and comprehensively instead of using only the number of processors to describe the parallel system. By introducing the concept of performance reference factor, it extends the conventional metrics to fit the SMP cluster architecture.As the experiments indicate, the extended metrics are applicable to the SMP cluster and have high precision.