The present invention provides a rubber composition which comprises a rubber component comprising 50% by mass or more of an isocyanate-modified diene rubber having a content of vinyl bond of 20% by mass or greater in the diene component and, based on 100 parts by mass of the rubber component, 2 to 20 parts by mass of a phenolic resin and 0.5 to 5 parts by mass of a curing agent for the resin and a run flat tire in which the rubber composition is used for the side reinforcing layer and/or the bead filler. Durability under the run flat condition is improved without adverse effects on ride quality during driving under the normal condition.