The computational performance of magnetohydrodynamic (MHD) code is evaluated on several scalar-type supercomputer systems. We have made performance tuning of a 3-D MHD code for space plasma simulations on the SR16000/L2, FX1, and HX600 supercomputer systems. For parallelization of the MHD code, we use four different methods, i.e., regular 1-D, 2-D, and 3-D domain decomposition methods and a cache-hit type of 3-D domain decomposition method. We found that the regular 3-D decomposition of the MHD model is suitable for HX600 system, and the cache-hit type of 3-D decomposition is suitable for SR16000/L2 and FX1 systems. As results of these runs, we achieved a performance efficiency of almost 20% for MHD code on all systems.