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Properties of natural rubber (NR) filled with various fillers, i.e., furnace black (N330), conductive carbon black (XE2-B), and carbon nanotube (CNT) were investigated. Both untreated and sonicated carbon nanotubes were used and designated as U-CNT and S-CNT, respectively. The filler content was varied from 0 to 8 phr. Regardless of the filler type, the increase in the filler content not only results in increased compound viscosity, reduced cure time, and enhanced cross-link density but also leads to the increase in the modulus and hardness of the vulcanizates. For N330 and XE2-B, the tensile strength increases continuously with increasing filler content. However, for both U-CNT and S-CNT, the tensile strength tends to increase with increasing filler content up to 2 phr and decreases noticeably afterward. At any given filler content, the CNTs give the vulcanizates with the highest values of electrical and thermal conductivities, storage modulus, and tan δ, followed by XE2-B and N330, respectively. Results also elucidate that the sonication of CNT without the presence of surfactant prior to mixing could not improve the degree of CNT dispersion, leading to insignificant difference in properties of the U-CNT-filled and S-CNT-filled vulcanizates. C