Equol is formed during the biotransformation of the naturally occurring isoflavone daidzein by intestinal bacteria. It has a stronger estrogenic effect than isoflavone. Therefore, equol is expected to affect the prevention of several hormone-dependent diseases such as prostate cancer, breast cancer, and menopause symptoms. However, the exact structure of the equol conjugates, for example, the position of conjugation sites, that circulate in vivo has not been determined. We synthesized equol metabolites and identified equol-G-’S in human plasma using LC-MS/MS by standard addition method. This paper describes equol metabolites analysis of the main metabolite in human plasma, and considers associated issues and prospects, with the focus on equol therapeutic potential.