We have carried out ab initio calculations on the electrical properties of Al chains with a substitutional Na impurity, focusing on the potential drop due to the applied bias voltage. Besides the expected electric current reduction by the introduction of Na impurity, we have found that the effective potential almost drops at the Na atom position wherever Na atom is located. This behavior is maintained when the applied bias voltage is increased. These results can be understood from the strength of interactions between atoms and between atom and electrode. [DOI: 10.1380/ejssnt.2006.570]