The effect of the tacticity of poly methyl methacrylate (PMMA) on the morphology of free volume in PMMA membranes was studied by using a molecular dynamics simulation technique. The chain flexibility, chain interaction, end-to-end distance of a chain and the time course of the free volume variation in the various stereo-regular PMMA, were obtained by an MD simulation technique. Simulation results depict that the construction of distinct tacticity has a significant influence on the flexibility of a molecular chain and the morphology of free volume in the syndiotactic (s-) and isotactic (i-) PMMA oligomers. MD calculation of the dynamic microstructure of a PMMA matrix shows that the s-PMMA membrane gives less flexibility in the backbone and a longer end-to-end distance than in the i-PMMA membrane. The less flexibility and longer end-to-end distance in the s-PMMA matrix causes the shapes and sizes of s-PMMA free volume to be longer and larger than those in an i-PMMA membrane. Furthermore, by adopting conformation energy minimization and molecular dynamics simulation techniques, various tacticity models of PMMA were constructed and their effect on the size of accessible free volume and free volume morphology was analyzed. Finally, the adsorbed gas on the surface free volume in s-PMMA was also compared with i-PMMA membrane.