Platinum nanoparticles stabilized by linear polyethyleneimine (LPEI) were prepared by the liquid-phase reduction of chloroplatinic(IV) acid with sodium borohydride. The average radii of particles were 3.26 and 1.76 nm when the molecular weights of LPEI were 25 000 and 2150, respectively. These nanoparticles were well dispersed in water in the pH range of 1-6. Branched polyethyleneimine also provided nanoparticles that dispersed in water in the pH range of 0-8. Linear poly(ethyleneimine-co-N-methylethyleneimine) gave nanoparticles that dispersed in water in the pH range of -1 to 15. The dispersibility of the nanoparticles decreased with increasing content of the N-methyl group.