Abstract: Efficiency and scalability are two important concepts for analyzing the performance of parallel computing systems. Although heterogeneous systems have become more and more common, the research of the heterogeneous system’s scalability returns very few back. This paper presents a definition of efficiency that can be fit for homogeneous and heterogeneous systems. According to this definition, it presents a scalability model that can be applied to both heterogeneous systems and homogeneous systems. Experimental results show that the model can estimate preferably the efficiency and scalability of parallel computing systems.