The mechanical properties of grain boundaries (GBs) and the development of grain-boundary sliding (GBS) were studied in the pure Zn using precision microindentation technique, optical, electron and atomic-force microscopy. Results have shown the different dependencies of the microhardness values on the indentation depth for GBs and individual grains. When the size of the plastic zone around the imprint was comparable to the grain size, GBs acted as barriers for dislocation sliding bands and twins. With applying the higher load, more grains were involved in the process of deformation, but microhardness did not increase. That was explained by the activation of GBS, leading to the relaxation processes. In its turn, the microhardness values measured at low loads in the vicinity to GBs and triple junctions (TJs) were higher than those measured in the grain interior. Thus, movement of the ensemble of defects to the GBs during microindentation is the activating factor for GBS in polycrystalline Zn. At the same time, during spreading of the deformation at low loads in the vicinity to GBs the activation of GBS was not observed.