The magneto-resistance effect of Co/Al-oxide/Co tunneling junction is evaluated by a first-principles band calculation. To clarify the effect of unoxidized Al in the Al-oxide layer on the Co layer, we calculated the electronic structures of the junction with a local density approximation using the LMTO-ASA method. Periodic superlattices which consist of layers separated by vacant layers were used for the conventional calculation method. The magneto-resistance ratio, Delta R/R, was estimated by the equation Delta R/R = 2P(1)P(2)/(P-1 + P-2), where P-1 is the polarization of a boundary Co atom on one side of a layer and P2 is the polarization of a Co or Al atom with a vacant layer on the other side. The polarization of Co atoms near the boundary between the Co and Al layers decreases greatly due to the paramagnetic Al, although the Al was polarized by the ferromagnetic Co. These changes of polarization affect the magneto-resistance ratio with calculations indicating that the ratio is reduced by half for two atomic layers of residual Al. (c) 2005 Elsevier B.V. All rights reserved.