The work function of Ir(100) surface coated with Cs overlayer is investigated by using jellium sphere/slab model containing three layers of Ir slab. A work function-coverage curve much reasonably closer to the experimental one is lbtained. The theoretical value of work function of clean surface is reduced to 5. 84 eV from the former result of 6.6 eV, which is in good agreement with experiment (5.5 eV). At the minimum, work function rises to 1.02 eV from former one (about 0.2 eV), which is much closer to the experimental result (about 1.3 eV). The relationship between the appearance of work function-coverage curve and the transfer of valence electrons in jellium sphere-vacuum region is analyzed.