The effect of R. oligosporus on structural and functional properties of fermented soybean was studied. After 24 hours of fermentation, the amount of total free amino acids formed greatly increased in soybean (1.20 g/100 g soybean). The pH of the fermented soybean was neutral during the growth of the mold. The amount of g-amino butyric acid (GABA) gradually increased during fermentation of the soybean (21.4 mg/100 g soybean per 24 hours). The proteins (albumin, globulin, alkaline soluble) were rapidly degraded to amino acids and low-molecular-weight peptides. Instrumental texture of fermented soybean (48 hours) had higher weakness (7.14 N), modulus of elasticity (1.02 × 10⁹ Pa) and surrender value (8.17 × 10⁶ Pa); these values were significantly different (P < 0.05) compared with 24 and 72 hours fermentation. Cell structure of fermented soybean was proportionally disorganized during the course of fermentation. The cell walls, cytoplasm, and vacuoles could not be distinctly identified. R. oligosporus produced enzymes which hydrolyzed protein, lipid, and starch, providing growth substrates and playing a role in its metabolism. Metabolism of mold was attributed to soybean cell disorganization, and the ratio of mature mold became higher, which was indicated by dark color.