In this paper, we use WCET(Worst-case execution time) tool, Bound-T, to analyze the key modules of typical real-time operating system (RTEMS and uClinux) affecting real-time performance. The WCET tool analyzes machine code to give the whole system's WCET before it runs on hardware. When WCET reaches our requirements, we evaluate the system real-time performance through benchmark. We compare the performance of two real-time operating systems and explain the advantages of RTEMS (Real Time Executive for Multiprocessor Systems).