silver mesocrystals have been synthesized via oriented attachment of primary building nanoparticles in simple and fast replacement reactions between AgNO3 solution and Sn. The formation process of silver mesocrystals is discussed in detail and the growth mechanism is suggested to describe the formation of silver mesocrystals. In a 200 mmol/L solution, the primary nanoparticles orientationally aggregate along <211> directions to form a dendrite, the proposed attachment planes and directions for Ag nanoparticles are 3×{422} planes and <211> directions, respectively. When the concentration increases to 1 mol/L, the primary building nanoparticles orientotionally aggregate along <211> directions to form dendrites. The dendrites that can be proposed as the second building units orientationally attach along <110> directions to construct a porous monocrystalline plate, and finally transit to a mesocrystal with a thickness about 50 nm. When the AgNO3 concentration is 2 mol/L, the building units are Ag triangle platelets. These platelets also orientationally attach along <110> directions to form a monocrystalline dense plate and finally the plate transforms to monocrystalline mesocrystal with a thickness over 100 nm