Viscosity is a first and foremost function both for theory and application. By approximating the interaction potential of molecule in fluidic fuel as constant and proposing a new method to calculate the momentum transfer by means of the radius of averaged volume per molecule, an equation of viscosity versus temperature was derived, which were found in good agreement with a lot of data from gaseous to liquid fuels by the curve fit. The equation also predicted that the viscosity initially decreased with temperature and then slowly increased in the high temperature region. When using it as correlation equation for the dependence of viscosity on temperature, it was able to cover more fluids from gaseous to liquid fuels. (C) 2015 Elsevier Ltd. All rights reserved.