The relative viscosity (RV) of polyvinylpyrrolidone (PVP) with different molecular weights was measured with a glass capillary viscometer and with a differential dual-capillary viscometer in water at different concentrations. For the differential dual-capillary viscometer, RV increases with a decreasing flow rate, especially for high molecular weight PVP at a 1% concentration. A good agreement in the RV between the two methods can be obtained for PVP with different molecular weights and at various concentrations if an appropriate flow rate is selected for the differential dual-capillary viscometer. Special precaution is needed when using the differential dual-capillary viscometer to measure the viscosity of a pure solvent.