Interfacially formed poly(N,N-dimethylaminoethyl methacrylate)/polysulfone (PDMAEMA/PSF) composite membranes were developed for CO2/N2 separation. A layer of PDMAEMA was deposited on a microporous PSF substrate by the solution coating technique, followed by crosslinking with p-xylylene dichloride (XDC) at the interface between the PDMAEMA solid layer and the crosslinking solution. The hydrophilicity and surface free energy of the membranes were analyzed by contact angle measurements with different probe liquids. The permselectivity of the membrane was shown to be affected by the PDMAEMA deposition time, interfacial crosslinking reaction time, and the PDMAEMA and XDC concentrations in the polymer coating solution and the crosslinking solution, respectively. The composite membrane showed a CO2 permeance of 85 GPU and a CO2/N2 ideal separation factor of 50 at 23 °C and 0.41 MPa of CO2 feed pressure.