Carbon blacks were used as catalysts for hydrogen production through hydrocarbon decomposition. The aim of this work is to find suitable conditions for decomposition reaction to cut down the net cost of hydrogen production. Carbon blacks after hydrocarbon decomposition under different operation conditions were mixed with NBR rubber. The surface area of carbon black increased with low weight gain in methane decomposition caused by carbon deposits on the surface of carbon black aggregates, and the decrease of surface area with further weight gain might be due to the carbon deposits adhering to each other and forming bigger aggregates. The same results were gotten from decomposition of mixture gas of methane and propane. The surface area of carbon black always decreased with the development of propane decomposition reaction. With the same carbon black loading, the composites filled by carbon blacks with low weight gain in methane and methane-propane mixture gas decompositions showed higher tensile strength than those mixed with raw carbon blacks, but there were no significant differences in 300% modulus. With the increase of carbon blacks loading in all composites, 300% modulus and tensile strength always increased. The surface resistivity of composites showed that it was much easier for carbon blacks with low weight gain in methane and methane-propane mixture gas decompositions to dissipate well in the in rubber system.