Nondispersive infrared (NDIR) gas sensor is the key component of vehicle exhaust testing system for CO and CO2, and the concentration of measured gas always is calculated by calibration method. The calibration curve generated by traditional calibration method can not fit very well when the gas concentration is lower than 10% of full scale and can not satisfy the measurement requirement. For this problem, a new calibration method based on weighted least square fit is presented. The method can improve the goodness of fit by giving bigger weight to data point at lower concentration and increasing the number of calibration points. The calibration result shows that the calibration curve generated by this method has relative error lower than 2% in the entire measuring scale. The calibration method presented is important in practical application.