(La0.67 Tb0.33 )2/3Sr1/3MnO3 has been studied in order to probe mechanisms responsible for the giant magnetoresistance ratios and the lattice effect in this kind of compound. The experiment has shown a strong connection between the magnetotransport and magnetovolume properties. An applied magnetic field not only gives rise to a large negative magnetoresistance (-900%) but also produces two different magnetovolume effects which reflect two different magnetostriction mechanisms in the compound.