Platinum nanoparticles were dispersed in mesopores of mesoporous silica using a sol-gel process with a composite template consisting of an amphiphilic triblock copolymer (Pluronic P123 or F127) and a Pt-organic complex, which was prepared with K2Pt(II)Cl4 as a Pt source and 1,10-phenanthroline as a chelating agent. The obtained Pt-1,10-phenanthroline complex did not dissolve in any of several solvents, e.g., hexane, benzene, toluene, THF, H2O, CH3OH, and C2H5OH. However, when the Pt-1,10-phenanthroline complex was reacted with ethylenediamine it dissolved in many solvents. Platinum nanoparticles dispersed in mesoporous silica were obtained using a sol-gel process with a complex template consisting of Pt-1,10-phenanthroline-ethylenediamine, and an amphiphilic triblock copolymer (Pluronic P123 or F127). A sample dried at 353 K was bright yellow. When it was subsequently heat-treated at 823 K, it turned light gray. This change indicates that Pt nanoparticles can be obtained by heat-treatment at high temperature, because, to generate Pt nanoparticles, the organics chelated to Pt ions must be removed. Measurements from small-angle x-ray scattering show that mesoporous silica obtained using a complex template has a much more highly ordered pore structure than that obtained using only an amphiphilic triblock copolymer. It has both large pores (above 8 nm) and a large surface area (about 290 m2/g). Furthermore, results of a TEM investigation showed that Pt nanoparticles were generated only in mesopores of mesoporous silica.