We performed first-principles calculations to study the stability of the alkali-atom adsorbed surface structure in Cu(001)-Li, Al(001)-Na and Ag(001)-Na systems. Our calculations predict that, at an alkali-atom coverage of 0.5, a substitutional (2x1) structure is achieved in Cu(001)-Li, whereas substitutional adsorption with c(2x2) may occur in Al(001)-Na and Ag(001)-Na. We calculated scanning-tunneling-microscope images of (2x1) and (3x3) structures in the Cu(001)-Li system using Tersoff and Hamann's prescription and compared them with recent experiments. In the (2x1) structure, the calculated result is consistent with the experimental data, where Li atoms appear transparent. In the (3x3) structure, Li atoms at the hollow site and in the trough are transparent when the sample bias is -1V. However, when a sample bias is +1V, a Li atom at the hollow site becomes visible.