One of the most important factors affecting the performance of turbine flowmeters is the fluid viscosity. On the basis of viscous fluid mechanics this paper presents a significantly improved theoretical model of turbine flowmeter performance. The calibration curves calculated by the present model show a better agreement with the results obtained in variable-viscosity experiments than the existing ones, thus providing the flow measurements by use of turbine meters in industries with a practical approach of viscosity-correction. The new model can alto be used to predict the effects of inlet velocity profile and meter geometry on the meter's performance, and thereby improve the design of the turbine flowmeter.