Abstract A series of polycrystalline bulk samples of (La 1− x Tb x ) 2/3Sr 1/3MnO 3 has been studied by means of X-ray diffraction, magnetostriction, thermal expansion, and magnetoresistance measurements. The average ionic radius of the A site 〈 r A〉, is systematically varied while keeping the carrier concentration fixed. It has been found that a phase transition from a rhombohedral to an orthorhombic form occurs at about the doping level x≅0.20 at room temperature accompanied by an anisotropic magnetostriction (AMS) up to −50×10 −6 under a magnetic field of 1 T. With increasing the terbium content x, the sharp increase of resistivity near the Curie temperature and the increase of the activation energies have been observed. The decrease in the intensity of the ferromagnetic double-exchange interaction has been correlated with the increase in the terbium content and the decrease of 〈 r A〉. The linear and volume magnetostrictions vary with chemical composition, even changing a sign. At low-temperature region ( T=80 K), the magnetostrictions for both the samples of x=0.20 and 0.40 exhibit different behaviors, i.e. the former has a positive volume and linear magnetostrictions and a negative AMS, while the latter has a contrary behavior. The magnitude of volume magnetostriction is large (∼10 −4) at T=80 K under a magnetic field of 4 T. We conclude that these anomalous effects are due to structural phase transition between orthorhombic and rhombohedral form and the charge delocalization induced by the applied magnetic field.