The reinforcement of sulfur-vulcanized natural rubber using cellulose nanofibers (CNFs) was investigated. Natural rubber consists mainly of cis-1,4-polyisoprene, and becomes stretchable after vulcanization. Vulcanization was performed via crosslinking with the polyisoprene double bonds, using sulfur and unsaturated fatty acids (oleic acid) incorporated on the CNF surfaces, resulting in highly efficient reinforcement of the CNFs. The Young's modulus of rubber reinforced with 5 wt% oleoyl was 27.7 MPa, i.e., 15 times higher than that of neat rubber, while retaining a strain to failure rate of 300%. The coefficient of thermal expansion of 226.1 ppm K⁻¹ for natural rubber was reduced to 18.6 ppm K⁻¹ after the addition of 5 wt% oleoyl CNFs.