Thin-film Au electrodes prepared by magnetron sputtering method were used to develop a potentiometric CO2 gas sensor. The thermal evaporated Li3PO4 played the role of electrolyte and sensing material. This design can simplify the fabrication process compared with the conventional solid electrolyte potentiometric CO2 sensor (c-sensor) and improve the performance compared with the thick-film Au electrodes sensor (t-sensor). The designed CO2 sensor (d-sensor) presented good response characteristics at the CO2 concentration range of 250–2500 ppm and the electromotive force (EMF), ΔEMF/dec, response and recovery time were investigated. The EMF values of the sensor were linearly dependent on logarithm of CO2partial pressure at the working temperatures between 420°C and 500°C. The d-sensor showed a sensitivity of 79.1–93.7 mV/dec at working temperatures range of 420–500°C which was much higher than the sensitivities of c-sensor and the t-sensor. The response and recovery time of the fabricated sensors were 10 and 14 s at working temperature of 500°C, respectively. The surface morphology of Au thin films was considered to increase the three-phase interface area and be good for the reaction gases to diffuse from Au film to lithium phosphate, allowing rapid chemical reaction equilibrium, getting a more stable EMF output.