In the present study, to clarify the reason why crude fiber digestibility is generally low in legumes than that in grasses, the digestibility of crude fiber in some grass and legume forages, and the digestibility of neutral detergent fiber（NDF）and acid detergent fiber（ADF）were also determined after refining them from same forages using an artificial rumen method. The following reasons why the digestibility of fiber of legume forages is usually lower than that of grass forages will be shown; a）There is a possibility that the constituents of cell except NDF control（prevent）NDF digestibility in legume forages. b）The digestibility of ADF itself is clearly low in legume forages as compared to that in grass forages. It can be also assumed as another reason that the digestion of hemi?cellulose proceeds rapidly, and will be degradable condition at early stage of incubation. It cannot be clarified, however that the rapid progress of hemi?cellulose digestion in the early stage during incubation induces a difference in digestibility of crude fiber between legume and grass forages, or not in this study.