High sensitivity chlorine (Cl2) gas detection has been realized by newly developed gas sensors using Cu-phthalocyanine (CuPc) thin films.The CuPc thin film gas sensors exhibited an increase of conductivity with exposure to Cl2 gas. The sensing properties of the CuPc thin film gas sensors were strongly dependent on the preparation conditions of CuPc thin films.The highest sensitivity for Cl2 gas was obtained in the gas sensor using CuPc thin film prepared under the optimized preparation conditions: substrate temperature of 170 ℃, evaporation temperature of 475 ℃ and film thickness of 50 nm.It was found that Cl2 gas detection was realized at a minimum concentration of 0.18 ppm.