The microscopic characteristics of brown layers in oak sawdust cultivation bed of Lentinula edodes was investigated with scanning electron microscope. Parenchymatous cells became thinner and the middle layers of the cells degraded in the oak sawdust bed at a plastic cultivation shed in four months. L. edodes hyphae penetrated into the pit and profusely branched within the bassal tube cells. Pit pores of ray parenchymatous cell in the decomposed sawdust became larger than in the uncolonized cells and were circular or irregular. The brown layer producing fruit body was 0.34㎜ thick and 0.67㎏/㎠ in hardness with especially 0.20㎜ thick layer of highly dense hypahe. On the other hand the brown layer without fruit body was 1.17㎜ thick, 0.94㎏/㎠ in hardness, and 0.80㎜ thick with smooth surface of highly compact dense hypae. This nonfruiting brown surface was heavily colonized with contaminating fungal hyphae and bacterial cells. We conclude that physical and biological characteristics of brown layer can affect fruiting of L. edodes.