A series of (La1-xTbx)2/aSr1/3MnOa polycrystalline samples has been studied by means of x-ray diffraction,magnetostriction, and thermal expansion measurements. It has been found that this series undergoes a phase transitionfrom a rhombohedral to an orthorhombic form at the doping level x≈0.20 at room temperature accompanied by ananisotropic magnetostriction up to -50×10-6 under a magnetic field of iT. The linear and volume magnetostrictionsvary with chemical composition, even change sign. At T=80K, the magnetostrictions for the samples of x=0.20 and0.40 exhibit different behaviours. The sample of x=0.20 has positive volume and linear magnetostrictions and a neg-ative anisotropic magnetostriction, while the sample of x=0.40 has an opposite behaviour. The magnitude of volumemagnetostriction for both the samples is essential (～ 10-4) at T=80K under a magnetic field of 4T. We conclude thatthese anomalous effects are due to the charge delocalization and the structural phase transition between orthorhombicand rhombohedral forms induced by the applied magnetic field.