Abstract: High strength, flexible and conductive composite elastomers for use as supercapacitor electrodes were synthesized using cellulose nanofibers (CNFs) and carbon nanotubes (CNTs) as the fillers, and natural rubber (NR) as the elastomer. An aqueous suspension of CNFs and CNTs was added to a natural rubber aqueous emulsion containing sulfur and a vulcanization accelerator, and the mixture was homogenized at a high-pressure, coprecipitated with a 1 M H₂SO₄ solution and vulcanized to obtain the composite elastomer. The microstructure, mechanical properties, electrical conductivity and electrochemical performance of the CNF-CNT/NR elastomers were investigated. Results showed that CNFs significantly improved the dispersion of CNTs in the aqueous suspension, leading to homogenous composite elastomers. The tensile strength and elastic modulus were significantly improved by adding CNFs. When the contents of CNFs and CNTs were 3 and 10 parts per 100 NR by weight, the tensile strength and elastic modulus of the composite reached maximum values of 6.44±0.32 and 8.77±0.48 MPa, respectively, and it was highly stretchable and flexible with a stable electrical conductivity of 1.78±0.86 S/m after stretching, bending and folding. It had a specific capacitance of 107 F/g at a current density of 0.3 A/g and retained 83% of the initial value at 1.0 A/g after cycling for 1200 times. This novel flexible, stretchable and conductive soft elastomer is useful in flexible electronic devices.