The synthesis of platinum ultrafine particles by the reduction of H2PtCl6 with hydrazine in AOT/isooctane reverse micellar solutions has been studied. By high-resolution electron microscope, electron diffraction pattern, and XRD analyses, the resultant particles have been found to be pure platinum of fee structure. Their sizes were observed to increase with the increases in the molar ratio of water to AOT (omega(O)) and in the concentration of H2PtCl6, while they decreased with the increase of hydrazine concentration. At a constant omega(O) value, the size of platinum ultrafine particles was not affected significantly when the concentration ratio of hydrazine to H2PtCl6 was above 10, the AOT concentration increased from 0.1 to 0.6 M, and the temperature varied from 15 to 35 degrees C. Furthermore, the kinetic study of particle formation indicated that the nucleation time needed several minutes. The time for the growth of platinum ultrafine particles to their final size after nucleation was about one to several hours. It was observed that the formation rates increased with the increase of omega(O) value and the concentrations of AOT and H2PtCl6, but they were not affected by hydrazine concentration when the concentration ratio of hydrazine to H2PtCl6 was above 10.