In this study, phytoestrogen for the industrial production of soybean probiotics by lactic acid bacteria (LAB) was studied in a soybean extract. Soybean was fermented with LAB, Lactobacillus plantarum KCTC 3108 and Bifidobacterum longum ATCC 15707. The change in the content of various isoflavones (aglycone and glucoside) and the $eta$-glucosidase activity in soybean during fermentation were investigated and shown to be dependent on the starter organism. Soybean extract powder fermented with L. plantarum showed the highest $eta$-glucosidase activity and the greatest increase in the aglycone content. After 48h of fermentation, the contents of daidzin, genistin and glycitin in L. plantarum decreased from a mean initial levels of $83.03{pm}2.17$, $168.13{pm}8.17$ and $20.02{pm}1.07$, respectively, to mean levels of $5.34{pm}3.24$, $3.79{pm}0.57$ and $1.87{pm}1.09;mg$/100 g. Whereas, after 48h fermentation, the contents of daidzein, genistein and glycitein increased from a mean initial levels of $8.09{pm}0.78$, $11.20{pm}0.84$ and $4.71{pm}0.46$, respectively, to mean levels of $85.76{pm}0.84$, $175.87{pm}2.21$ and $22.41{pm}0.91;mg$/100 g. Taken together, these results suggested an increase of aglycones and decrease of glucoside in isoflavones occurred during fermentation, which coincided with an increase of $eta$-glucosidase activity in the fermented soybean extract powder.