In this study, the effect of rheological unit on Carbon Black (CB) dispersion was investigated using several mercaptan modified Chloroprene Rubber (CR) with different viscosities and crystallizations. It is known that CR has three different states depending on temperature during mixing, which are elastic phase, granular phase and plastic phase from the low temperature side. In addition, rheological units are fine particles with a size of about 1 ƒÊm or less, which are considered to be generated by subdivision in mastication. As a result, the prolongation of mastication of CR in elastic phase improved CB dispersion, although there was little change in viscoelastic properties and molecular weight distribution curves. The increase in the specific surface area of rheological units during mastication seems to contribute to this improvement. On the other hand, the elapsed time of masticated CR from mastication to mixing worsened CB dispersion. The decrease in the specific surface area due to the reaggregation of rheological units after mastication will affect this decline. These indicate seemingly that rheological units generate in CR due to subdivision during mastication. Furthermore, it was found that lower temperature of rubber between addition of CB and maximum electric power, even within temperature range of elastic phase, improves CB dispersion more.