A model is developed to account for the viscous flow of glass-forming liquids. We consider that a supercooled glass-forming liquid is composed of a liquid phase and some solidlike clusters which have a higher activation energy for viscous flow than the liquid regions. This approach yields a fitting equation for viscosity with a simple form eta=eta(0) exp(E/k(B)T)exp(PhiT/T-g), where E is the activation energy for viscous flow at low temperatures, and Phi represents the fragility of a glass-forming system. Our results suggest that kinetic viscous flow of glass-forming liquids is closely related to their thermodynamics.