A polyethylene oxide-b-polystyrene (PEO-b-PS) block copolymer incorporating UV-crosslinkable coumarin groups in the PS block self-assembled into a cylindrical structure with PEO cylinders perpendicular to the film surface, which exhibited excellent CO 2 separation properties. The block copolymer was successfully synthesized by atom transfer radical polymerization (ATRP). The molecular characterization of the diblock copolymer was performed with 1H nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). The UV-crosslinking of the film was monitored by UV-vis absorption spectroscopy. The cylindrical phase structure was confirmed by transmission electron microscopy (TEM). Gas permeation properties of CO 2, N 2 and He were determined at different temperatures varying from 20 °C to 70 °C. Both the CO 2 permeation flux and total gas selectivity increased with increasing temperature. The maximum of CO 2 permeance at 70 °C was 20400 × 10 -6 cm 3 cm -2 s -1 cmHg -1, and gas selectivity over He and N 2 was 20.1 and 27.7, respectively. It was concluded that the functional block units and self-assembled microphase structures synergetically played key roles in the high performance of the membrane.