The influence of mastication of natural rubber (NR) on properties of carbon black-filled NR compounds was studied. Variations of viscosities of the masticated NR and the carbon black-filled NR compound by heating were also investigated. The viscosities of the compounds decreased by increasing the mastication time of the raw rubber. The viscosities of the masticated NR and the compounds increased by increasing the heating time. This was explained by the combination reaction between functional groups in the rubber. The viscosity increment of the masticated rubber and the compounds by heating became larger with increased mastication time. Cure rate of the compound became faster by increasing the mastication time. Modulus of the vulcanizate made of the rubber masticated for a long time was higher than that of the vulcanizate made of the rubber masticated for a short time while elongation at break of the former was shorter than that of the latter. This was explained by the content of bound rubber and chain length of the rubber molecules.