Abstract: Rotational viscometer with coaxial cylinder sensor system is a major instrument to measure the viscosity of crude oil. Generally, the measurement is made under the non-airtight condition, as a result, volatilization of the light component in the oil specimen will cause bias to the measurement results. Such problem can be solved by using airtight system. However, airtight system can also be used to measure the fluid viscosity under high temperature and high pressure. Error analysis is made based on the structure of the viscometer and the principle of measurement. A method of varying shear rate is brought forward to measure the viscosity of crude oil by using the VT500-D100/300 system. Significant improvement of reliability and repeatability of viscosity measurement is obtained by using this method.