Abstract: Scalability has become an important consideration in parallel algorithms and machine designs. Two important scalability metrics: the isoefficiency function and the isospeed metric are rewiewed in the paper. The merit and deficiencies of these two metrics are pointed out. Then based on the analysis of the parallel execution time, a new metric （isoratio of parallel overhead to computation） is proposed. The new metric can be used to measure and evaluate the scalability of parallel algorithms and architectures. Finally,the new metric is used to analyze the scalability of two parallel algorithms and the YH 3 MPP supercomputer.