Thick-film CO2 sensors were fabricated using a sodium super ionic conductor (NASICON: Na1+xZr2SixP3-xO12) with a series of Li2CO3-BaCO3 auxiliary phases. The powder of a precursor of NASICON with high purity was synthesized by a sol-gel process. Using the NASICON paste, an electrolyte was prepared on the alumina substrate through screen printing and then sintered at 1000°C for 5 hours. The resulted NASICON thick film was characterized by XRD and FESEM. Within a wide range of CO2 volume ratio concentration from 1,000 to 10,000 ppm, the output of the sensor showed a good electromotive force (EMF) response that was very close to the theoretical value. The sensor attached with a binary carbonate auxiliary phase Li2CO3-BaCO3 (1:2) showed fairly good CO2 sensing properties at 470°C.