The temperature dependence of the viscosity of a normal nonpolymeric liquid is analyzed theoretically based on a free volume model. We show that the viscosity is mainly a function of the activation energy for molecular collision. The activation energy is closely related to the temperature, and the relation between the two can be derived based on a cell theory. The applicability of the result derived is justified by fitting it to the experimental data for n-paraffins. We show that, in addition to providing a physically more realistic analytical expression than those in the literature, the analytical result derived in our study is capable of improving the performance of the empirical relation proposed by Doolittle [J. Appl. Phys. 22, 1031 (1951)]. The applicability of the present model to predict the basic properties of glass-forming liquids is also illustrated. (C) 2003 American Institute of Physics.