Abstract: Five trials on the ϕ300 mm Smith crude oil P. D flowmeters have been made in a metering station, and, the flowmeter's factors at discharge capacity of 300 m³/h, 500 m³/h, 700 m³/h, and 900 m³/h have been obtained. On the basis of these factors, the relation law of change of flowmeter factors/ tear and wear/ oil loss has been analyzed. It is pointed out that the insulated-room method (the flowmeter is installed inside) can not prevent the prover from wax depositing, therefore, the hot water around 65 ℃ should be used to clean the prover, and the full deliquescence wax and pigging should be done prior to flowmeter's calibration to avoid wax deposit.