Scanning Microscopic light scattering is a kind of photon correlation spectroscopy, which was used to study the internal homogeneity of cross-linked EPDM. The samples were swollen with toluene for several days to insure the equilibrium state. The equilibrium swollen samples with toluene were characterized by SMILS, laboratory dynamic light scattering equipment. The dynamic properties of the EPDM samples were studied. The cooperative diffusion coefficient D_coop was determined which comes from Brownian motion. The correlation length of internal three dimensional network structures was done from Stokes-Einstein relation, which actually represents the mesh size of polymer network. Experimental results show that the network size decreases as a function of the cross - linked amount. It is observed that the variations of D_coop with cross-linked concentration. The SMILS data of mesh size were compared with those calculated from the swelling measurements. It is found the good agreement between both data.