In this study, polyurethanes (PUs) and polyurethane ureas (PUUs) based on 4,4'-diphenylmethane diisocyanate (MDI) with poly(tetramethylene oxide) (PTMO), poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), propylene oxide-ethylene oxide triblock copolymer (PEO-PPO-PEO), poly(dimethylsiloxane) (PDMS), and mixed PDMS/PTMO, PDMS/PEO, PDMS/PPO, and PDMS/PEO-PPO-PEO soft segments were synthesized in order to investigate their structures-gas permeation property relations. The mixed PDMS/polyether copolymer membranes were characterized by Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) measurements. TEM showed that the mixed PDMS/polyether copolymer membranes had more phase-separated structures than PUs and PUUs having single soft segments. For gas separation such as O-2/N-2 and CO2/N-2, the small addition of PDMS into polyether-based PU matrices led to both gas permeabilities and selectivities Of O-2/N-2 and CO2/N-2. This might be that the incorporation of PDMS leads to the phase separation in both hard segment (MDL/BD) and soft segment (polyethers) due to the difference of solubility parameter, and the dispersed PDMS phases serve to produce a more tortuous route for diffusing molecules. While the small addition of polyethers into PDMS-based PUU matrix decreased gas permeabilities, and did not affect the selectivity of O-2/N-2 but increased the selectivity of CO2/N-2.