The effect of cobalt salt in rubber compound on the physical properties of the rubber compound and the adhesion properties between the rubber compound and brass-coated steel cord was studied. Although the change in the physical properties of the rubber compound with the addition of cobalt salt was small, the adhesion properties were changed significantly with the loading level of cobalt salt and aging treatments. Pullout force and rubber coverage of the steel cord were enhanced in the rubber compound with low loading of cobalt salt. When the loading level of cobalt salt was higher then 1.5phr, the adhesion was severely deteriorated after aging treatments. The easy failure in the adhe-sion of the sample with high loading of cobalt salt was explained in terms of the growth of the interfacial layer.