The relationship among Co/sub 100-x/Fe/sub x//Al-oxide/Co/sub 100-x/Fe/sub x/ tunnel junctions (x=0, 10, 25, 40 and 50) and tunneling magnetoresistance (MR) was investigated. The highest MR ratio of 40.7% was observed for Co/sub 75/Fe/sub 24//Al-oxide/Co/sub 75/Fe/sub 25/ junctions. The MR ratios for Co/sub 75/Fe/sub 24//Al-oxide/Co/sub 75/Fe/sub 25/ junctions were almost constant as a function of junction area, which implies that those junctions that possess the MR ratios over 40% do not show geometrical enhancement effect on the MR ratios. After annealing at 200/spl deg/C, MR ratio of 49% was obtained for the Co/sub 75/Fe/sub 24//Al-oxide/Co/sub 75/Fe/sub 25/ junctions. When the bias voltage was between 360 and 455 mV, the MR ratio decreased to half its value at 1 mV. MR ratio was observed over 20% at 400 mV For Co/sub 75/Fe/sub 25//Al-oxide/Co/sub 75/Fe/sub 25/ junctions. The bias voltage dependence of MR ratio was related to the barrier height of the junctions. MR ratio decreased more slowly with increasing bias voltage for those junctions with high barrier heights.