A remarkable CER(colossal electroresistance)effect was observed in a series of polycrystalline Nd0.67Sr0.33MnO3 samples synthesized by high energy ball milling successive heat treatment.The CER ratio increased with increasing heating time and reached 3580% at a low temperature for the Nd0.67Sr0.33MnO3 ceramic annealed up to 13 h as measuring current was enhanced from 5 to 500 μA.The result showed that the CER effect was strongly dependent on the sample's microstructure,especially on the foreign phases which were gradually developed within grain boundaries when annealing time was more than 3 h.