Equol, a microbial metabolite of daidzein, has been hypothesized as clue to the effectiveness of soy and its isoflavones but only 33% - 50% of human population can excrete equol. Recently studies indicated that hydrogen gas playsan important role in equol formation. However, researches focus on this area are just started. In this paper, the corresponding research results of previous studies have been reviewed, including the underlying mechanism of equol formation and bioavailability, factors influencing equol production and the role of hydrogen gas on microbial equol formation.