As natural gas viscosity is one of the most important parameters in natural gas engineering calculations, its accurate value determination plays a key role in its management. In this study, a comprehensive model is suggested for prediction of natural gas viscosity in a wide range of pressures (14.69-20053 psia), temperatures (434-820 degrees R), and gas specific gravity of 0.573-1.207. The new model is applicable for gases containing heptane plus and non-hydrocarbon components. It is validated by the 3255 viscosity data from 25 different gas mixtures. The average absolute error of the model was found to be 3.03% and 5.89% in the comparison with the natural gas and gas condensate viscosity data respectively, compared to existing similar methods, its results are quite satisfactory. (C) 2013 Elsevier B.V. All rights reserved.