Carbon blacks were used as catalysts for hydrogen production through hydrocarbon decomposition under different operation conditions such as temperature, residence time, and hydrocarbon precursor. Those modified carbon blacks were used as reinforcement fillers in both NBR and SBR rubbers. The specific surface area of carbon black increased with low weight gain in decomposition of methane and mixture gas of methane-propane caused by the protruded carbon deposits on the surface of carbon black aggregates but the decrease of specific surface area with further weight gain occurred probably due to the carbon deposits adhering to each other. At the same loading ratio, composites filled by carbon blacks with low weight gain in methane-propane mixture gas and methane decomposition reactions exhibit overall improved mechanical properties. Modulus at 300 % strain measured showed superior values for SBR rubber composites filled by carbon blacks after propane decompositions. The surface resistivity of composites reveals that more developed filler network of carbon blacks inside the rubber system could be obtained after decomposition of methane-propane mixture gas and methane with low weight gain.