The structure of Co ultrathin films deposited on 1 ML Ag/Pt(1 11) surface was studied by Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). The atomic exchange between Ag and Co occurs at a high temperature annealing. The magnetic property was studied by a magneto-optic Kerr effect (MOKE). The easy axis of the magnetization of Co deposited on 1 ML Ag/Pt(111) switches from the out-of-plane to the in-plane direction when the coverage of Co is larger than 2 ML. The easy axis can change back to the out-of-plane direction after a high temperature annealing for 2-7 ML Co/1 ML Ag/Pt(111). The study of the interface structure shows that the spin reorientation transition occurs when the atomic exchange between Ag and Co occurs. Further annealing at higher temperatures, the perpendicular magnetic anisotropy significantly enhances. The enhancement of the perpendicular magnetic anisotropy is due to the formation of Co-Pt alloy after the atomic exchange between Co and Ag. The possible mechanisms for the spin reorientation transition and the enhancement in the perpendicular magnetic anisotropy are discussed. (C) 2003 Elsevier B.V. All rights reserved.