This study presents a new method for measuring size distributions of colloidal nanoparticles in a liquid dispersion. This method is based on a photon correlation spectroscopy (PCS) combined with a flow cell and a size exclusion chromatography (SEC). The SEC unit classifies injected nanoparticles into several monodisperse nanoparticles. The classified nanoparticles introduced from the SEC unit are analyzed sequentially by the PCS unit. The size distribution of the polydisperse mixture was calculated by integration of all the monodisperse distributions. The size distributions obtained using the new method were compared with those obtained by scanning electron microscope (SEM) image analysis. For the three kinds of colloidal silica nanoparticles used in this study, good agreement was found, verifying the utility of the methodology.