Cheonggukjang, a traditional soy food, was prepared with addition of Lentinus edodes powder at 0, 5, and 10% (w/w), and the distribution of isoflavones were analyzed by high performance liquid chromatography during fermentation for 48 h. ${eta}$-Glucosidase activity and changes in succinyl genistin and succinyl daidzin were monitored. ${eta}$-Glucosidase activity in cheonggukjang with 5 and 10% (w/w) L. edodes powder were significantly higher than those in control samples (p <0.05). Total isoflavones in 48-h fermented cheonggukjang with 0, 5, and 10% (w/w) L. edodes powder were 4.88, 4.26, and 3.99 ${mu}mole/g$, respectively. Aglycones of isoflavones in cheonggukjang with 5 and 10% (w/w) L. edodes powder were 27.61 and 24.76% for 24 h and 28.2 and 38.74% for 48 h, whereas those in control samples were 5.50 and 21.11% for 24 and 48 h, respectively. Succinyl daidzin and succinyl genistin in L. edodes powder-added cheonggukjang were significantly lower than those of control samples (p <0.05), implying that ${eta}$-glucosidase activity from L. edodes negatively affected the formation of succinyl derivatives.