It is necessary to reduce the viscosity of heavy crude oil in many cases such as transportation and production of crude oil. One of the methods to reduce the viscosity is the addition of lighter oils. There have been many models to calculate the viscosity of the mixed oil. Unfortunately, there is no universal model for the computation. In this paper, five frequently-used models were chosen and evaluated. Totally 20 mixed oil samples were prepared with different ratios of light to crude oil from different oil wells but the same oil field. The viscosities of the mixtures under the same shear rates of 10 s(-1) were measured using a rotation viscometer at the temperatures ranging from 30 degrees C to 120 degrees C. After comparing all of the experimental data with the corresponding model values, the best one of the five models for this oil field was determined. Using the experimental data, one model with a better accuracy than the existing models was developed to calculate the viscosity of mixed oils. Another model was derived to predict the viscosity of mixed oils at different temperatures and different values of mixing ratio of light to heavy oil. (C) 2011 Elsevier Ltd. All rights reserved.