In this thesis, a surfactant-free Np-KCl matrix method (Np stands for nanoparticle, KCl is potassium chloride) is developed for the synthesis of nanoparticles with controlled size and structure. In this method, a Np-KCl composite is formed in a onepot reduction in THF at room temperature. KCl is an insoluble by-product 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 oC. 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 can be released from the KCl matrix in an ethylene glycol-water solution and transferred to support materials forming a uniform Np-support composite. An in-situ STEM (scanning transmission electron microscope) study revealed that the mechanism of nanoparticle growth during annealing is dominated by particle-particle coalescence, although Ostwald ripening also occurs at a slower rate. A detailed study of the synthesis of ordered intermetallic Pt3Fe and PtFe nanoparticles with an average diameter of 4 nm is provided as two examples of the method. The generality of the method was extended to synthesis of other bi- and tri-metallic nanoparticles of platinum transition metal compounds. We expect that a wide variety of non-Pt metal containing nanoparticles can be also prepared using this same approach.