It was observed that a phenolic plant product modified carbon black exhibited significant improvement of flex crack growth resistance over the control unmodified carbon black. At a certain ratio of sulfur to accelerator i.e. 2:1, vulcanizates showed maximum flex crack growth resistance. This was confirmed after aging at 100 degrees C. The higher retention for the modified black filled recipe is explained by the attachment of a flexible hydrocarbon moiety with carbon black in the course of modification.