Poly(alkyl methacrylate-co-sodium methacrylate) ionomer membranes with different alkyl side chain length were prepared and performances were characterized by the permeability method with poly(ethylene glycol) (PEG) 200 and 1000 aqueous solution to investigate the structural effects of polymer materials on the separation performances of membrane. The flux and rejection of PEG 200, PEG 1000 aqueous solution for the ionomer membrane showed maximum values at a certain side chain length of the ionomer (maximum flux and rejection of PEG 1000 is 15.5 kg/m(2) h, 84.5%, respectively). The number of water molecules participating in the hydration shell surrounding the ionic aggregates was decreased with increasing the side chain length of the ionomer, The volume of the hydration shell decreased and the flexibility of the matrix polymer chain increased with increasing the side chain length of the ionomer membrane. The separation performance of the ionomer membrane shows optimum value due to the enhanced flexibility of matrix polymer and the reduced volume of hydration shell with the side chain length of the ionomer membrane because the flux of PEG solution increases while the rejection decreases with increasing the volume of hydration shell and the flexibility of matrix polymer chain. Both the flux and the rejection were increased with increasing the molecular weight of the solute, The surface interaction between the solute and the membrane was postulated to mainly affect the separation characteristics.