The effect of stearic acid loading in rubber on the adhesion between rubber compounds and brass-plated steel cords was examined in respect to aging treatments and cure conditions. The adhesion interphase of a thin brass film/rubber compound containing stearic acid was investigated using an Auger electron spectrometer. The loading of stearic acid to rubber was not effective in the improvement of adhesion of rubber stock cured under optimum conditions, whereas it improved the adhesion property of premature cured samples by enhancing the physical property. High loading of stearic acid severely deteriorated the adhesion property, even at an unaged state due to the excessive formation of zinc sulfide. The loading of stearic acid did not affect the sensitivity of the rubber compound to humidity and thermal aging, but amplified the deterioration of adhesion when the rubber compound was subjected to salt solution aging, resulting in extremely poor adhesion. The effects of other ingredients and the aging treatment were also thoroughly examined.