Abstract: The pipe prover is the standard instrument to calibrate the flowmeter, which is used in the in-line dynamic measurement of oil. To decrease the indication errors of the flowmeter and increase the precision of oil measurement, the accuracy of the pipe prover should be increased at first. Based on the principle of wax deposition, it is inferred that the wax will deposit on the internal surface of prover when ambient temperature and pipe wall temperature are low, which will influence the indication errors of flowmeter of in-line calibration to different extent. Through analysis on the extent of wax deposition, the different thicknesses of wax layer can be obtained. In order to decrease the prover errors caused by ambient temperature, the pipe prover should be installed indoors, and its standard section must be insulated so as to increase the ambient temperature and to make the wall temperature of the standard section equal or higher to the oil temperature. In addition, more calibrating pigs should be launched under the peak flowrate before calibrating. The verification should not be started until the temperature, pressue and flowrate are all stable. By means of these, the measurement errors due to wax depostion can be eliminated, the accuracy of custoday transfer will be increased.