The adhesion between a rubber compound containing epoxidized natural rubber (ENR) and a brass-plated steel cord was studied to shed light on the role of ENR for the improvement of adhesion. The addition of ENR in the rubber compound caused cure rate to decrease, and as ENR loading increased, modulus and hardness increased while tensile strength and elongation at break decreased. An improvement of adhesion was obvious with low loading of ENR at the range of 2 phr, while an adverse effect was observed with higher concentrations of 10 phr. Since the ratio of sulfur to copper at the interphase of the rubber compound/brass film adhesion sample increases with increased ENR loading up to 2 phr, the acceleration of sulfide formation by ENR could be proven. Adequate curing of rubber and the formation of high-molecular layer at the interphase by addition of small amount of ENR improve adhesion. However, the toughening of the rubber caused by the addition of larger amounts of ENR degrades the adhesion due to the poor physical properties of the rubber compound.