We prepared tunnel magnetoresistance(TMR) devices of Ta($50AA$)/NiFe($50AA$)/IrMn(150$AA$)/CoFe($50AA$)/Al ($13AA$)-O/CoFe($40AA$)/NiFe($400AA$)/Ta(50$AA$) structure which has 100$ imes$100 $mu extrm{m}^2$ junction area on $2.5Times2.5 cm^{2}$ $Si/SiO_2$ ($1000AA$) substrates by a inductively coupled plasma(ICP) magnetron sputter. We fabricated the insulating layer using a ICP plasma oxidation method by varying oxidation time from 80 sec to 360 sec, and measured resistances and magnetoresistance(MR) ratios of TMR devices. We used a high resolution transmission electron microscope(HRTEM) to investigate microstructural evolution of insulating layer. The average resistance of devices increased from 16.38 $Omega$ to 1018 $Omega$ while MR ratio decreased from 30.31 %(25.18 %) to 15.01 %(14.97 %) as oxidation time increased from 80 sec to 360 sec. The values in brackets are calculated values considering geometry effect. By comparing cross-sectional TEM images of 220 sec and 360 sec-oxidation time, we found that insulating layer of 360 sec-oxidized was 30 % and 40% greater than that of 150 sec-oxidized in thickness and thickness variation, respectively. Therefore, we assumed that increase of thickness variation with oxidation time is major reason of MR decrease. The resistance of 80 sec-oxidized specimen was 160 k$Omega$$mu extrm{m}^2$ which is appropriate for industrial needs of magnetic random access memory(MRAM) application.