Abstract: When measuring crude oil using flow meter plus on -line densimeter, the mass of crude oil shall equal to the product of its volume, density and a conbined correction factor. Volume of the crude oil is measured by a flow meter, and density by a on -line densimeter. Introduced in this paper is the measurement principle of NMZ dual string densimeter of vibration type, the precision of which is influenced by the medium measured, ambient temperature, liquid pressure, flow rate as well as liquid viscosity. Proper correction of the factor based on the knowledge on the characteristics of densimeter can achieve a limited measuring error as low as± 0. 001 g/cm³