The present invention provides a rubber composition for coating a steel cord in a belt layer which can decrease a decreasing rate of tensile properties caused by deterioration of thermal oxidation, inhibiting peeling from the steel cord and further reducing a rubber flow, and a steel cord coated with a carcass layer comprising the same. The present invention relates to a rubber composition for coating a steel cord comprising (A) at least 3 parts by weight and less than 5 parts by weight of sulfur, (B) 0.1 to 1 part by weight of hexamethylenebisthiosulfate disodium salt dihydrate, and (C) a cobalt salt of an organic acid containing boron based on 100 parts by weight of a rubber component, wherein the amount of cobalt in the cobalt salt of a boron containing organic acid (C) is 0.05 to 0.25 part by weight based on 100 parts by weight of the rubber component, and a steel cord coated with a carcass layer comprising the same.