The tunnel magnetoresistance(TMR) ratio have been investigated in the Fe/Al oxide/Fe junctions, specifically the temperature and applied voltage dependence of the TMR ratio. It was found that the dependence of resistance at saturation magnetization state(Rs) on the applied voltage at 4.2 K is completely anomalous in terms of the expected tunneling behavior. This "zero-bias anomaly" is due to the magnetic impurity in or near the boundary between ferromagnetic electrodes and insulator. The TMR ratio decreased rapidly with increasing temperature. The TMR ratio also decreased when increasing the applied voltage. We discussed the dependence of the TMR ratio on temperature and applied voltage taking into account the impurity assisted tunneling.