We present a local density functional calculation of the effect of an external electric field on the work function change of Pd and Ag adsorption on an Al(001) surface. The adsorption of K has also been considered for comparison. We found that the work functions for all the systems increased linearly when the strength of the external electric field was increased. Since the polarized dlectrons at the interstitial regions between the adsorbate and substrate for Pd/Al(001) and Ag/Al(001) react to the external electric field differently, the subtle differences between Pd/Al(001 and Ag/Al(001) bondings has been characterized through the comparison of the slopes of the work function change versus electric field.