A rubber composition for reinforcing a side having both low exothermic property and high hardness is provided.The invention relates to a rubber composition for reinforcing a side of a run flat tire, comprising 10 to 100 parts by weight of carbon black having a nitrogen adsorbing-specific area of 30 to 100 m.sup.2/g and a dibutyl phthalate oil absorption amount of at least 50 ml/100 g, at least 2 parts by weight of sulfur or a sulfur compound, and 5 to 120 parts by weight of (1) a lamellar alumina powder or (2) fluorine-containing mica having an aspect ratio of 3 to 50 based on 100 parts by weight of a diene rubber component.