The effects of solvent types on accessible free volume and dynamic conformation of polymer chains in atactic poly(methyl methacrylate)s (a-PMMA)s were studied by molecular simulation techniques. Five well-minimized amorphous packing models were constructed to form a-PMMA membranes with different casting solvents by utilizing the amorphous builder module of Cerius(2) Software. The fractional free volume, fractional accessible free volume and free volume size distribution of PMMA membranes in various solvent environments were analyzed. The solubility coefficients were also calculated for oxygen and nitrogen by using the Grand Canonical Monte Carlo (GCMC) method. Simulated results indicated that the gas diffusivity of the membrane is mainly affected by free volume and accessible volume, while the gas solubility is highly dependent on the free volume size distribution at the interface.