We report an improved synthesis of colloidal Ag(n) nanoprisms using carboxyl compounds (citrate or succinate) and long chain macromolecules (polyvinylpyrrolidone (PVP)). The side-facet structure of the triangular nanostructure was determined in detail using electron tomography in scanning transmission mode (3D STEM) and HRTEM. It has been found that they are built up by {100} facets with a single parallel twin plane. The best conditions for producing uniform Ag nanoprisms with tunable sizes and high yields in the presence of carboxyl compounds additive system are described, and a growth mechanism is proposed. This approach provides also a route to synthesize Ag nanodisks and Au-Ag alloyed nanoprisms.