Parallel programming is essential for large-scale scientific simulations, and MPI is intended to be a de facto standard API for this kind of programming. Since MPI has several functions that exhibit similar behaviors, programmers often have difficulty in choosing the appropriate function for their programs. An MPI benchmark program library named MBL has been developed for gathering performance data for various MPI functions. It measures the performance of MPI-1 and MPI-2 functions under several communication patterns. MBL has been applied to the measurement of MPI performance in the Earth Simulator. It is confirmed that a maximum throughput of 11.7GB/s is obtained in inter-node communications in the Earth Simulator.