We report a reducing agent-free method for preparing platinum (Pt) nanoparticles by internally isopropanol (IPA)-modified dendritic poly(amido amine) (PAMAM). The internally modified dendritic PAMAM were synthesized via divergent strategy using 1,3-diaminopropanol as a linking spacer, and NMR analyses confirm the embedded IPA moieties within the dendrimers by the appearance of characteristic proton and carbon resonances at 3.81 and 67.9 ppm, respectively. The in situ formation of stable Pt colloids was carried out by thermal treatment in the presence of internally modified dendritic PAMAM bearing either ester or alcohol peripherals, suggesting that the internal IPA functionalities dominate the reduction of Pt4+ ions. Moreover, the overall reducing rate was accelerated with increasing pH values. This result agreed with a reaction feature for preparing metal nanocomposites through polyol process in which basic environment facilitates the thermal-promoted reduction of metal ions accompanied with the oxidation of internal hydroxyl groups. The morphology of the dendrimer/Pt composite monitored by a transmission electron microscope (TEM) exhibited narrowly dispersed and roughly spherical shaped nanoparticles with a mean diameter of 5.4 nm.