Nanocrystalline silver (Ag) was prepared by in situ compacting of ultra-fine silver particles : The structures of as-compacted and annealed specimens were analysed by high-resolution transmission electron microscopy and scanning electron microscopy. Vickers microhardness was measured on the specimens. The ultra-fine particles aggregate before compaction. It is found that the nanocrystalline specimens are obtained by the compaction of the aggregates. Microstructure inside the aggregates does not change as the compacting pressure increases from 0.25 to 2.00 GPa. The compacting pressure affects on the structure and density of the boundaries between the aggregates, i.e. the formation of the crack-type defects of about 1 mu m at the boundaries. Thermal stability of nanocrystalline Ag is significantly low; grain coarsening starts below 200 degrees C. However, a nanometre-sized layered structure forms in local regions upon annealing and is stable up to 800 degrees C. Vickers microhardness of as-compacted specimens increases with increasing compacting pressure. The increase is attributed to the decrease of the number of crack-type defects. Vickers microhardness of nanocrystalline Ag begins to decrease due to grain coarsening upon annealing around 200 degrees C. The microhardness of nanocrystalline Ag deviates from the Hall-Fetch relation.