To investigate thermal destruction and rearrangement of the carbon black (CB) network consisting of CB aggregates in rubbery matrix, the proper heat-treatment temperature without thermal decomposition of rubber matrix were examined by using differential scanning calorimetry, dynamic mechanical measurements, and thermal expansion measurements. 383 K was chosen as the heat-treatment temperature under vacuum. The volume resistivity (rho(v)) of 50 phr CB-filled natural rubber vulcanizate (CB-50) increased rapidly up to a heat-treatment time of 24 h and it decreased by further heat-treatment time, whereas the rho(v) of CB-80 remained almost constant without depending on heating time. Three-dimensional electron microscope (3D-TEM) observations revealed that after the heat-treatment for 75 h, the average lengths of the crosslinked and the branched chains and the crosslinked points density (D-cross) of the CB network decreased, whereas the branched points density (D-branch) increased with decrease of D-cross. After the heat-treatment, their fractions (F-cross's) of crosslinked chains decreased, whereas their fractions (F-branch's) of the branched ones increased. Especially, F-branch of CB-50 became larger than that of CB-80, while the decrease of F-cross of CB-50 was almost the same as that of CB-80 by the heat-treatment. And, F-cross and F-branch of the heat-treated CB-50 were the same compositions (F-cross and F-branch = ca. 0.7 and ca. 0.3, respectively) as those of the heat-treated CB-80. It is suggested that the CB network of CB-80 is more thermal stable than that of CB-50. These results directly indicate that CB network is broken and is rearranged by a heat-treatment. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 122: 1300-1315, 2011