The work-function change of potassium on the Cu(111) surface was studied. The work-function change was measured by a Kelvin probe. The coverage of potassium on the surface was determined by AES. Maximum work-function change Δφmax=3.14 eV. Initial dipole moment p(0) = 7.0 Debye, The minimum work-function occurs at the coverage θmin= 0.18. The result of this work is consistent with that of the jellium-slab model.