Using Pt3Fe nanoparticles as an example, a surfactant free Np-KCl matrix method (Np stands for nanoparticle) is developed for the synthesis of nanoparticles with controlled size and structure. In this method, the Np-KCl assembly is formed in a one-pot reduction in THF at room temperature. KCl is an insoluble byproduct of the reaction and serves as a matrix that traps the nanoparticles to avoid particle agglomeration and to control the coalescence of nanoparticles during thermal annealing up to 600 degrees C. By varying the molar ratio of metal precursors and KCl, as well as the time and temperature of annealing, the final particle sizes and crystalline order can be independently controlled. After thermal processing, nanoparticles were released from the KCl matrix and transferred in an ethylene glycol-water solution to support materials forming a uniform Np-support assembly. A detailed study of the synthesis of ordered intermetallic Pt3Fe nanoparticles with an average diameter of 4 nm, using this Np-KCl method, is provided as an example of a generally applicable method. This surfactant free strategy has been extended to the synthesis of other bi- and trimetallic nanoparticles of Pt-transition metals.