Viscoelastic solids with high viscosity were experimentally studied with the intention of having them deformed under uniform shear stress. A kind of sandwich method was developed for applying a constant shear stress to a specimen, and its deformation was observed optically using heterodyne interferometry with a sensitivity of 10 nm in displacement measurement. Time‐dependent deformation data were analyzed on the basis of a mechanical model of anelasticity plus viscosity. Viscosity in the range of 108–1014Pa⋅s could be determined at temperatures of 20–200 °C. Through a simulation using the finite element method together with an experiment visualizing the deformation, the specimen deformation was shown to be of an almost uniform shear mode. Experiments were performed to determine the temperature dependence of viscosity for several kinds of glasses near their glass transitionTg, and the determined viscosity values were about 107Pa⋅s atTg. The viscosity values measured by a rotation disk viscometer were in good agreement with the present data. The viscosity of some glasses was also measured by the beam bending and penetration methods, and values of 109–1012Pa⋅s atTgwere obtained. The large discrepancy between the two kinds of data was considered. ©1996 American Institute of Physics.