The annealing effects on the magnetic properties, crystallographic structure, and alloy formation, were studied for Co/Pt (111) ultrathin films at coverages up to 6.6 ML, using in situ magneto-optical Kerr effect, low energy electron diffraction, Auger electron spectroscopy,and ultra-Violet photoelectron spectroscopy. After the postdeposition annealing in the temperature range. of 500-800 K, a significant increase in perpendicular magnetic anisotropy at high coverages and a large enhancement of magneto-optical response with the value about 200%-300% of that before annealing for all coverages investigated are observed. Both findings are shown to be correlated to the formation of a kind of Co-Pt interface (surface) alloy. This is expected to be mainly attributed to the transfer of strong spin-orbit coupling of the Pt into the Co magnetic orbital due to the hybridization during interface alloy formation.