Some interesting magnetic properties were observed after Ag ultrathin films were deposited on Co/Pt(111). The spin reversal of Co can be prevented when Ag atoms cap Co on Pt(111). The out-of-plane magnetization has a significant enhancement after annealing at high temperature. The Curie temperature increases from 625 to 710 K as 1 ML of Ag is deposited on 1 ML Co/Pt(111). The change of surface anisotropy and the formation of a Co-Pt alloy are possible mechanisms for the magnetization enhancement.