The effect of plasma-radical change on the surface properties of Zn-Mg-Al ternary-alloy-coated steel sheets during atmospheric-pressure (AP) plasma treatment using different process gases: O ₂, N ₂, and compressed air was investigated. The plasma-induced radicals promoted the formation of chemical particles on the surface of the Zn-Mg-Al coating, thereby increasing the surface roughness. The surface energy was calculated using the Owen-Wendtgeometric equation. Contact angle measurements indicated that the surface free energy of the alloy sheets increased upon AP plasma treatment. The surface properties of the Zn-Mg-Al coating changed more significantly in the order air > O ₂ > N ₂ gas, indicating that the plasma radicals facilitated the carbonization and hydroxylation of the Mg and Al components during the AP plasma treatment.