Relative efficiency of different types of fillers e.g., phenolic resin, carbon black and carbon black plus phenolic resin hybrid system in nitrile rubber vulcanizates for improvement of physico-mechanical properties, resistance to swelling in oil/fuel and thermal stability has been studied. The hybrid system was always found to offer maximum benefit in strength and dynamic mechanical properties coupled with higher percent of (a) retention of physical properties due to aging and (b) swelling resistance in oil or fuel. Both thermogravimetric analysis (TGA) as well as thermomechanical analysis (TMA) reflect much better thermal stability for the combined filler system compared to the vulcanizates containing either phenolic resin or carbon black.