Abstract: Thermocouple is a temperature sensing device,which made the temperature into a voltage. Through electrical measuring instrument to measure the thermal potential value, the calibration process is completed and the result is obtained. There will be uncertainty in this result inevitably. According to the theory of assessing and expressing the uncertainty, in allusion to the factors influencing the results of the calibration process, the source of uncertainty in calibrating Pt-Rh-10 thermocouple (S graduation) was analyzed. The standard uncertainty from the components was assessed,then the expanded uncertainty of the mensurement results was obtained. One ensures that the calibration process in strict accordance with the standard method to measure, reduce or eliminate the influence of random factors,improving the reliability of the measurement of marriage.