Abstract: A rubber composition comprises 10 to 70 parts by mass of carbon black having 20 to 100 m2/g of the specific surface area by nitrogen adsorption per 100 parts by mass of a rubber component comprising 20 to 80 parts by mass of a modified polybutadiene rubber and 80 to 20 parts by mass of natural rubber and/or at least one other diene-based synthetic rubber, wherein the modified polybutadiene is obtained by polymerizing 1,3-butadiene in an organic solvent using a compound having a rare earth element of the lanthanoid series, followed by modifying the obtained polybutadiene, and has a content of the cis-1,4-bond of 92% or greater, a content of vinyl bond of 1.5% or smaller and a fraction of the modified chain end of 20% or greater; and a tire using the rubber composition for a sidewall. The rubber composition exhibits excellent low heat buildup property and resistance to fracture.