After postdeposition annealing in the temperature range of 500-800 K, a highly enhanced magneto-optical Kerr response up to a maximal value 300% of that before annealing, is observed for 1-2 ML ultrathin Co-Pt (1 1 1) films. With help of low energy electron diffraction and Anger electron spectroscopy, this enhancement of magneto-optical response is found to be correlated to the Co-Pt alloy formation at interface.