MTJs of structure Si/SiO2/Ta/Ru/IrMn/CoFe/Ru/CoFe/Al-O/CoFe/NiFe/Ru with different surface roughness of bottom electrode were prepared by sputtering, and it was investigated that the dependence of TMR ratio and resistance-area product (RA) on plasma oxidation time, thickness and surface roughness of tunnel barrier, and the tunneling characteristics of junction devices through I-V curves. To get resistance of below RA 10 kΩμm2, oxidation time of 10 s for 8 Å thick Al layer was required. In this case, thickness of Al2O3 barrier layer was 12.5 ~ 14 Å. For the 13 Å thick Al2O3 tunnel barrier, TMR ratio of optimized MTJ with uniform tunnel barrier was about 45% at bias voltage of 100 mV. Also the barrier height and the barrier width fitted to Simmon's relation were 12.3 Å and 3.07 eV, respectively, and these values agreed with that of MTJ within error range. The I-V curve and TMR ratio versus bias voltage curve of MTJ with rough tunnel barrier were linear and asymmetric, respectively, but in case of MTJ with uniform tunnel barrier, these curves were non-linear and symmetric, respectively. It was confirmed that the smooth surface of bottom electrode was a basic requirement for MTJ.