Abstract: The calibration principle for the positive displacement flowmeter is introduced and the calibrating error is analysed. The calibrating error is mainly due to the wax deposited on the internal surface of prover, which makes the standard volume of prover reduction; and the working temperature of standard prover is mistaken during operating, which produces wrong readings. In order to increase the calibrating accuracy for the flowmeter, the paper suggests that for the wax, 4 -5 calibrating pigs should be lauched to scrape the wax before calibrating; for the temperature, the three methods should be adopted: ① insulating the prover; ② improving the survey method for temperature; ③ the actual temperature of the prover equalling approxi-matlly the half as large as the sum temperature of oil temperature and room temperature.