Gold and platinum nanoparticles were synthesized by the reduction of their salts with potassium bitartrate as the reductant and poly(N-vinyl-2-pyrrolidone) (PVP), polyethylene glycol (PEG) (for Pt nanoparticles) or 3,3′-thiodipropionic acid (TDPC) as the protective agent. In the presence of PVP or TDPC, the preparation of palladium and silver nanoparticles were also realized by the reduction of their salts with potassium bitartrate under alkaline conditions. The effect of the concentration of the protective agents on the particle size and morphology were investigated in detail. At a constant concentration of Au and Pt ions, the average diameter of Au or Pt nanoparticles tends to decrease with an increase in the concentration ratios of the protective agents to AuCl₄⁻ or PtCl₆²⁻. Except for PVP passivated Au nanoparticles, smaller Au and Pt particles with a sharp size distribution could be obtained at higher concentration ratios of the protective agents to the metal ions, as determined by TEM measurements. All the noble metal colloidal particles have very high stability.