Surface modification by segregation is a potential method to control the work function. Property of surface with segregation is expected to be the same as that with adsorption. The saturated behavior of surface composition in segregation phenomena is added advantage over adsorption. The segregation behavior of Cu on Ti film/Cu substrate was studied by heating the specimen in ultra high vacuum and the change in work function for different concentrations of segregated Cu on Ti film was measured with a Kelvin probe. Work function decreased linearly with increasing segregated Cu concentration and the amount of decrease in work function is about 0.3 eV for 5 at.% Cu at saturation.