An in situ dielectric measurement for atactic poly(methyl methaerylate) (at-PMMA) was performed under high-pressure CO2 under various pressures and temperatures. The at-PMMA has the acetate side group with a large dipole moment. In the glassy state, a local relaxation process (beta-process) can be observed using dielectric measurement. In the rubbery state, the micro-Brownian motion of main chain (alpha-process) occurs, and the beta-process changes into alpha beta-process coordinated with the alpha-process. The dielectric loss (epsilon") spectrum of at-PMMA in the glassy state is asymmetric because of the density fluctuation for the amorphous structure. The loss peak frequency shifted to higher frequencies, and the relaxation strength increased with increasing CO2 pressure. In the glassy state, the shape of epsilon" spectrum became more symmetric with increasing CO2 pressure. These show that the molecular mobility enhanced by the plasticization effect Of CO2 allows the dipolar side groups in the high-density region to contribute to the relaxation process. We also found that the apparent activation energy decreased under high-pressure CO2. (c) 2005 Wiley Periodicals, Inc.