Molecular simulation techniques were adopted to investigate membrane free volume morphologies and gas-transport mechanisms in the aromatic polyimide (PI) membranes composed of various diamines and dianhydrides. A molecular dynamics (MD) technique was adopted to analyze the fractional free volume (FFV), fractional accessible volume (FAV), free Volume size and shape, and diffusion mechanisms. A Monte Carlo (MC) method was used to analyze the gas sorption behaviors in the membranes. The FFV, FAV, and free volume morphology analyses reveal that bulky groups in the PI membranes contributed to the formation of a larger and more continuous free volume. The thermal motion analysis shows that a greater effective free volume in the membratics promoted effective motion, such as jumping and diffusive motions. The sorption analysis indicates that the larger free volume provides more sites for gas molecule absorption. The MID and MC results provide good agreement with the experimental data from past reports, which validates the feasibility of molecular simulation techniques in gas separation membranes at a molecular scale.