Abstract: The results ultrasonic measurement of wall thickness was important influent factor for grading-detection of in-use oil well tubing. In order to evaluate the uncertainty of wall thickness measurement, based on the method of ultrasonic measurement of wall thickness of oil well tubing, mathematical model was built, component of uncertainty were identified and analyzed. By simplified calculation, the uncertainty of measurement with setting confidence rate was put forward, and the implication of measurement result expressed was declared concisely. By calculating and analyzing, the evaluation step of the uncertainty of ultrasonic measurement of wall thickness of in-use oil well tubing was built.