Basic parameters characterizing silicon nitride viscosity have been obtained from experiments on Si3N4 films. Changes in film thickness and intrinsic stress after various types of high-temperature annealing have been studied. A considerable stress increase during a temperature ramp-up is attributed to film structure densification. The contribution of the ramp-up and ramp-down stages of annealing to film shrinkage was excluded from the calculations of nitride viscosity. The resulting viscosity is higher than previously reported. Based on the experimentally obtained viscosity values, the process of local oxidation of silicon (LOCOS) was simulated. The viscosity of thermal oxide and stress-dependent parameters of the viscoelastic oxidation model were extracted by comparing the simulated boundary geometry of LOGOS structures with cross-sectional scanning electron microscopy images. It is found that the enhancement of oxidant diffusion in oxide regions subjected to tensile stress is a major cause for the increase in pad oxide thickness under a narrow nitride mask during field oxidation.