The formation of a polycrystalline solid compact, by fusing an initially porous aggregate of diamond crystals under dynamic shock pressure (7.5–18 GPa), is shown to depend critically on the size of the initial crystals. Porous aggregates of 100–150 &mgr;m diameter crystals upon shock compaction produced compacts. These exhibited pronounced fracturing of the individual crystals and showed no evidence of fusion. Aggregates consisting of ultrafine crystals (<5 &mgr;m) also exhibited minimal consolidation. However, samples composed of crystals in the range 4–8 &mgr;m produced strong fused compacts of polycrystalline diamond. A model calculation indicates that at 10 GPa less than 0.07 mass fraction of the diamond powder can be melted and this molten material is quenched in 0.8 ns for 8‐&mgr;m‐diam crystals.