The present invention provides a rubber composition a tire, by which both low rolling resistance and wet grip performance can be achieved at high levels. This rubber composition for a tire is obtained by blending from 66 to 180 parts by weight of a silica having a nitrogen adsorption specific surface area of from 150 to 400 m2/g into 100 parts by weight of a diene rubber; a size Rss of aggregates of the silica forming a hierarchical structure being from 14 to 23 nm when the rubber composition is measured by ultra-small-angle X-ray scattering and an obtained scattering profile is applied to a Unified-Guinier function.