Thick film CO2 gas-sensor using La2O3-SnO2 material system was fabricated by coating method where SnO2 thick film was coated by LaCl3-7H2O solution and heat-treated in air. The starting material LaCl3.7H2O of La-coating solution was phase-transformed from LaCl3 to LaOCl at temperature range of 367-738$^{circ}C$ and from LaOCl to La2O3 over 773$^{circ}C$ The CO2 sensitivity of sensor increased wit increase of heat-treatment temperature. The highest sensitivity to CO2 gas appeared at heat-treatment temperature of 100$0^{circ}C$ but the CO2 sensitivity decreased at the temperature higher than 100$0^{circ}C$ The decrease of CO2 sensitivity over 100$0^{circ}C$ was because the grain growth of SnO2 happened and thus resulted in the reduction of surface area of SnO2 thick film even though the amount of La2O3 phase having most superior sensing characteristics among La compounds increased at high temperature. The highest senstivity of sensor to CO2 gas also appeared at La concentration of 0.01M