A study was carried out about the effect of free fatty acids and lecithin on the thermal inactivation of horseradish peroxidase. In presence of lecithin peroxidase was inactivated very rapidly at 70℃ and pH 7.0, and showed also a rapid inactivation curve at 0℃ and pH 4.0. Linoleic acid was more effective in an O₂-stream than in a N₂-stream, but oleic acid showed a similar tendency in the presence of O₂-and N₂ stream. From the results of the experiment we suggested that the opening of the heme crevis of peroxidase in the presence of lecithin and the produced lipidperoxide from linoleic acid may accelerate the thermal inactivation of horseradish peroxidase respectively.