Abstract: The constant volume and conductance method calibration system is a metrology standard for low range throughput.The calibration system is composed the constant volume flowmeter and the conductance method.The constant volume flowmeter can measure more gas flow and the conductance method can measure small gas flow.The constant volume flowmeter has two work modes.It can measured inflow or outflow gas flow from the constant volume.The vacuum leaker can be calibrated with the constant volume and conductance method.The calibration range is 5×10~ -2~5×10~ -11Pa·m~3/s,the combined standard uncertainty is 0.56%~1.70%.