Gold nanoparticles were synthesized by means of a reduction method based on the two-phase method reported by Brust et al. [J. Chem. Soc. Chem. Commun., 801-802 (1994)]. The size of the particles was investigated as a function of the reduction rate of the gold ions. The reduction rate was controlled by changing the rate at which reducing agent was added to the gold salt solution. The result showed that the size was changed by the reduction rate. It was found by transmission electron microscopy observation that the lower the reduction rate, the greater the mean diameter of bold nanoparticles. The mean diameter varied from 1.7 to 2.1 nm. This change was also seen in the UV-vis absorption spectra and X-ray diffraction patterns. We demonstrated that the mean diameter of bold nanoparticles is influenced in the range of angstroms by reduction rate.