We synthesized Pt nanoparticles encapsulated in poly(amidoamine) (PAMAM) dendrimers by Pt2+ coordination and subsequent reduction by NaBH4. To optimize the experimental conditions for the Pt nanoparticle synthesis, we systematically examined the effects of pH, temperature, coordination time, and surface functional groups of the dendrimers on coordination and NaBH4 reduction by UV-vis spectroscopy and transmission electron microscopy (TEM) measurements. We used generation-4 dendrimers (hydroxyl-terminated PAMAM dendrimers; G4-OH) and generation-4.5 dendrimers (carboxyl-terminated PAMAM dendrimers; G4.5-COO-). According to our results, dendrimer-encapsulated Pt nanoparticles with a narrow size distribution were obtained at high pt(2+) coordination ratios (alpha), while nonencapsulated Pt nanoparticles were formed at low alpha values. To enhance alpha, it was necessary to use a neutral G4-OH solution or an acidic G4.5-COO-solution. Temperature had a marked effect on the coordination rate, with an increase in the temperature from room temperature to 50 degrees C, and the coordination time decreased from 10 days to 1-2 days.