Ferromagnetic tunneling junctions with Al-oxide and Ta-oxide as insulating layers were fabricated and the microstructure of the oxide layers was investigated with cross-sectional transmission electron microscopy (TEM). TEM analysis showed that as the Al-oxide thickness increased from 13 to 63 Angstrom, the roughness rapidly increased and the magnetoresistive (MR) ratio also markedly dropped. The undulation of the Al-oxide with increasing thickness was proved to have resulted from the plasma oxidation process. In contrast, Ta-oxide layer remained flat regardless of the layer thickness. In comparison to the Al-oxide layer, partially oxidized Ta-layer unction exhibited poor MR characteristics. Growth mechanism for Al- and Ta-oxide layers microstructure is proposed in terms of oxidation kinetics and oxygen plasma characteristics. (C) 2002 Elsevier Science B.V. All rights reserved.