Abstract: A CO/CO2 gas sensor was developed at low temperature, which was composed of Sn, SnF2|LaF3(mixed)|Pt (Ni), i.e., taking LaF3 (mixed) monocrystal or polycrystal as base material, Sn and SnF2 as reference electrode and Pt wire mesh as working electrode. It was used to measure separately the two gases CO and CO2 at 295.15-343.15 K for the relationships of EMF to volumetric fraction and to temperature under conditions that volumetric fractions were in the range from 5% to 30% (with Ar taken as gaseous diluent), as well as the response time needed for equilibrium EMF the cell just reach. The experimental results show a similarity between both gases that the EMF of cell increases with the increase of volumetric fraction at a fixed temperature and vise versa. In addition, the EMF value of monocrystal is higher than that of polycrystal for the same gas under the same conditions including volumetric fraction and temperature. The response time of both gases are about 2-5 min.