The author proposes a new concept with a new interface model for carbon black reinforcement of elastomer, based on stress analysis. The new model consists of double uncrosslinked polymer layers of different molecular mobility, the inner glassy hard (GH) layer, and the outer sticky hard (SH) layer surrounding a carbon particle. In this report, the most essential and fundamental three subjects in the carbon black reinforcement are discussed. Large stress increase with filler content and increasing strain amplitude results from the strong stress concentration generated around carbon particles and its transmission to the whole system. The great increase in tensile stress is only possible when the stress-hardened superstructure produced in the SH layer under large extension supports the large stress concentration. The super structure is the network of carbon particles interconnected by strands of oriented and extended molecules, together with a craze-like phenomenon forming numerous microvoids. Stress softening, called the Mullins effect, mainly results from the buckling of the strand of oriented molecules. Thus, the large and instant stress reduction takes place in unloading, in which the main load-bearing force is the entropic, contractile force of matrix crosslinked rubber. During long periods, the extended molecules in the uncrosslinked SH layer will considerably relax and return to their original entropic state. (C) 2004 Wiley Periodicals, Inc.