Dried tempeh is known to be remarkably stable to lipid oxidation. An isoflavone has been isolated and is considered to be one of the antioxidants in tempeh. However, the true origin of the antioxidative activity of tempeh is still obscure. In this paper, isoflavones and their glucosides in tempeh were analyzed by HPLC, and the liberation of isoflavones from glucosides occurred during fermentation was made clear. The main isoflavones responsible for the antioxidative activity in tempeh were deduced to be daidzein and genistein.