Abstract: The principle, structure of equipment, operating method and main characteristics of ultrasonic contact impedance (UCI) hardness test were introduced. The UCI hardness tester was used to determine the excircle surface hardness of a carburized and quenched roller part. The main sources of measurement uncertainty affecting the test results were analyzed and the measurement uncertainty of the testing results was evaluated. The results show that the UCI hardness of the roller part was H_UCI=(63.2±1.6) HRC when the coverage factor k=2. The main sources of measurement uncertainty for UCI hardness test were the harness conversion and the repeatability error of measurement. In practical test, Vickers hardness was recommended as the output hardness in order to minimize the uncertainty of harness conversion. It was also recommended to use special tools or assistive devices to stabilize the test process and reduce repeatability error of measurement.