An experimental study using dielectric spectroscopy measurements is made oil molecular motions as functions of temperature and pressure of polystyrene (PS) and poly(vinyl methyl ether) (PVME) mixtures having large difference in glass transition temperatures. For mixtures with high concentration of PS, the largest loss peak and a shoulder at higher temperatures are observed. Our detailed analysis of molecular motion leads to the result that the former is not due to the alpha process of PVME-rich region arising from dynamic heterogeneity for miscible polymer mixtures, but due to local cooperative motion where favorable orientational motion of PVME is induced through a change in the environments formed by the PS matrix.