An experimental study was performed to investigate the effects of deviatoric stress on the microstructure of partially molten polycrystalline aggregates. Borneol (organic crystal) + melt system having an eutectic temperature of 43¿C and a moderate dihedral angle was used as a partially molten rock analogue. Large samples (70 mm cube) having melt fractions of 0.089--0.22 were deformed ductilely under a uniform pure shear stress (shear strain rate of ~0.8--7¿10-7 s-1), while monitoring the sample microstructure in situ using ultrasonic shear waves. Each sample was deformed repeatedly by changing the principal stress direction, resulting in the microstructural changes well detectable under the microscope. The most remarkable features of the stress- and/or deformation-induced microstructural changes are enhanced grain boundary wetting, enhanced grain coarsening, and formation of large (approximately sample scale) melt sheet parallel to the shear plane, which is an assembly of completely wetted two-grain boundaries at the microscopic scale. Significant changes in the elastic, anelastic, and viscous properties associated with these microstructural changes were observed. The velocity monitoring provided a detailed picture of growth and healing of grain boundary melt films in response to loading cycles. The microstructural changes observed in this study have many similarities with the "dynamic wetting" reported for the partially molten peridotite.