This paper is concerned with the discussion on the characterization of carbon-gel developed in the carbon black filled rubber systems. A limit of NMR method for the characterization was also discussed. There exists less mobile rubber molecules and highly mobile ones in the carbon-gel. The less mobile rubber molecules cannot be removed by the good solvent for rubber and they contribute to the reinforcement. The highly mobile ones can be extracted by the good solvent depending on molecular weight of rubber and conditions for the extraction. Therefore, the reinforcement of filled rubber systems cannot be simply related with the content of carbon-gel.