Abstract: Future supercomputing demands that large scale parallel algorithms and applications have good scalability. Previous scalability studies lay stress on the studies of the algorithms scalability,but few on that of the application programs. They couldn’t give users the information about how to adjust programs to improve its performance. The numerical scalability and parallel scalability are provided to describe whether the parallel system maintains its numerical attributes and parallel attributes. Furthermore, a suit of scalability evaluation criterion is provided to help the user to find the reason causing the bad scalability and to modify programs. This criterion and the near optimal scalability method are used to analyze the scalability of a large scale application program, namely two\|dimensional electromagnetic plasma with particle in cell method. Results show that the criteria help to locate the reason why the scalability is bad, and that the near optimal scalability method provides an approach to predict how many processors are to be used by a larger problem to get a reasonable utility, where its time is near to the shortest time to run and its efficiency is much improved.