The data measured by small angle X-ray scattering (SAXS) and the extinction spectra observed by spectroscopy were examined as the in situ measurements of the size distribution of gold nanoparticles and were compared with the results obtained from the photographs by a transmission electron microscope (TEM). The mean size can be estimated from the wavelength of the peak absorbance using Mie theory. However, the particle size distribution can only be estimated from fitting the calculated spectrum to the experimental one of the sample with above 30 rim diameter. On the other hand, the scattering data measured by a SAXS instrument can estimate the mean size from the slope of the Guinier plot, when the particle size distribution is narrow. In addition, the size distribution and mean size can be calculated by the experimental SAXS data when the log-normal or Schulz-Flory size distributions are assumed.