Abstract: This invention provides a rubber composition capable of improving a durability to heat buildup of a heavy duty pneumatic radial tire by applying to a cushion rubber of the tire to lower heat buildup of the cushion rubber without lowering a fracture strength of the cushion rubber as well as a tire using such a rubber composition and being excellent in the durability to heat buildup. The rubber composition is characterized in that 25-45 parts by mass of carbon black having a nitrogen adsorption specific surface area (N2SA) of not less than 60 m2/g as a filler is compounded based on 100 parts by mass of a rubber component comprising at least one of natural rubber and synthetic isoprene rubber, and a modulus at 50% elongation is not less than 1.1 MPa, and a loss tangent (tan ?) at 25° C. and 2% strain and an amount of carbon black compounded based on 100 parts by mass of the rubber component satisfy a relation of the following equation (I): tan ?/amount of carbon black compounded?0.0015??(I).