This study aimed to investigate the effects of grape leaf extract (GLE) on the inhibition of lettuce polyphenol oxidase (PPO) and on the loss of phenolic compounds in fresh-cut lettuce during storage. The addition of GLE that had been heated at 100C for 15 min exhibited a higher inhibitory effect on the lettuce PPO activity than that of the fresh extract. It was found that heat-treated GLE inhibited the lettuce PPO noncompetitively. The addition of GLE prevented loss of lightness in lettuce monitored during storage. Loss of phenolic compounds was measured during the enzymatic browning process by high-performance liquid chromatography. Protocatechuic acid, chlorogenic acid, ferulic acid, coumaric acid and caffeic acid were identified in lettuce. Degradation of phenolics followed the first-order kinetic pattern during storage. Synergistic effects of GLE, in combination with ascorbic acid or citric acid, were also tested for the protection of phenolics.