The distribution ratio of carbon black (CB) in a blend of polyisoprene rubber (IR) and polybutadiene rubber (BR) is characterized using high-resolution solid-state C-13 NMR. The relationships between the line width of the polymer's resonance lines and CB contents are estimated for both the IR/CB and BR/CB composites from the dipolar decoupling/magic angle spinning (DD/MAS) 13C NMR spectra. For the IR/CB composite, the IR line width increases with the increasing CB content. A similar relationship is obtained between the BR line width and CB contents for the BR/CB composite. These relationships are used as calibration curves. On the basis of these calibration Curves, the distribution ratio of CB in the IR/BR rubber blend is determined. It is found for the IR/ BR/CB composite that the amount of CB in the BR phase is approximately twice as high as that of CB in the IR phase, irrespective of the CB content. The sum of the amounts of CB in both the IR and BR phases, determined from the above C-13 NMR experiments, mostly agree with the amount of CB originally added to the compound. This verifies the validity of the CB distribution determination method by C-13 NMR, as proposed in this study. A comparison with a CB-gel method, which can only be used for urivulcanized rubber blends, further confirms the results obtained from the C-13 NMR.