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 CO₂ 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 CO₂, N₂ and He were determined at different temperatures varying from 20 °C to 70 °C. Both the CO₂ permeation flux and total gas selectivity increased with increasing temperature. The maximum of CO₂ permeance at 70 °C was 20400 × 10⁻⁶ cm³ cm⁻² s⁻¹ cmHg⁻¹, and gas selectivity over He and N₂ 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.