The glass transition temperature (T-g) reduction of polymer by plasticization is a paramount thermodynamic property in polymer/diluent systems, thus effecting microcellular foaming process. Experiment was conducted in a high-pressure cell furnished with an optical channel. In this research, the swelling ratios (Delta V/V-0) of polymethyl methacrylate (PMMA) in carbon dioxide (CO2) with pressures ranging from 2 to 22 MPa were measured, thus the T-g data of PMMA was determined in the same pressure range. It was found that the swelling ratio increases with temperature but reaches the maximum at 12 MPa, instead of 22 MPa. And the glass transition temperature decreases from 380.5 to 311.4 K within the pressure range of 0.1-12 MPa but increases gradually to 323.1 K from 12 to 22 MPa. Additionally, the change of maximum saturation temperature as a function of CO2 pressures agrees well with T-g of the polymer-CO2 system. (C) 2016 Elsevier B.V. All rights reserved.