Composite membrane with superior performance can be employed for CO2 separation from flue gas. The separation performance of a composite membrane can be improved by fine-tuning the selective layer and porous substrate. The optimal substrate should have suitable interfacial property and surface morphology, as well as minimal gas permeation resistance. The present study discusses the approach to improve the hydrophilicity and morphology of nanoporous polyethersulfone (PES) substrates by incorporating a small amount of hydrophilic additives, such as sulfonated polysulfone (SPSf), hydroxylated polyethersulfone (PES-OH), and polyvinylpyrrolidone (PVP). The surface and bulk morphologies of the PES substrate were modified to reduce the gas permeation resistance by incorporation of the hydrophilic additives. Moreover, a thinner selective layer containing hydrophilic polymers can be coated on top of the improved PES substrates due to the enhanced hydrophilicity. Both the morphological and hydrophilic modifications can improve the separation performance of the composite membrane. The developed PES substrates described in this paper are suitable for the preparation of composite membrane in gas separation. A resistance-in-series model was employed to evaluate the effect of modified substrates on separation performance improvement. It was found that, with a defined selective layer, the separation performance of a composite membrane was affected by the lateral diffusion on the selective layer-substrate interface and the Knudsen diffusion in the nanoporous substrate. For the selective-layer material in this study, the data fitting showed that 25-30% of the total resistance was caused by the lateral diffusion, while 5% of the total resistance was due to the Knudsen diffusion in the substrate.