The relationship between molecular structure and zero-shear viscosity of polymers was studied. In this study we propose a new equation, which is based on Berry and Fox's equation. This new equation is constructed from some molecular parameters, such as mean square length and average molecular weight of statistical skeletal unit, characteristic ratio, entanglement molecular weight, glass-transition temperature, free volume fraction at glass-transition temperature, and thermal expansion coefficient of free volume. It is proposed that some of these molecular parameters could be predicted by group contribution methods, except for the free volume parameters. We also propose new empirical relations between free volume parameters and molecular structures of polymers, which make it possible for free volume parameters to be obtained from molecular structure. Using these relationships, it is possible that the zero-shear viscosity and its temperature dependence are obtainable from the molecular structure of polymers. We applied this formula to some polymers, including both amorphous and semicrystalline polymers. Comparison between the measured and calculated zero-shear viscosity showed quite good agreement.