Much effort has been studied in the determination of the particle size, using either chemical or physical method. The chemical methods are based on the selective chemisorption of one probe molecule and, the main physical techniques are based on the direct observation by electron microscopy or by X-ray diffraction (XRD) line broadening. They allow to estimating mean particles size. In this study, the various techniques of particle size measurement of Pt and Au has been determined by chemisorption of CO, XRD and TEM.