To investigate the effect of sample size addition into the fermentation flasks using Lee`s modified method (1971) on the organic matter digestibility of alfalfa hay and barley straw, 0.25gm, 0.5gm, 0.75gm and 1.0gm of each forage were placed into separate fermentation flasks containing 75㎖ of rumen liquor and McDughall`s buffer solution. There was a significant difference (P$lt;0.05) between 0.25gm and 1.0gm of sample size treatment. Since there was no significant difference between treatment 0.5gm and 0.75gm of sample size, either of which could be adopted for the in vitro fermentation method for further experiments. A significant (P$lt;0.05) difference was found between the sample size of alfalfa and that of barley straw, but there was no significant difference in interaction between the kind of feedstuffs and the sample sizes. It was confirmed that Lee`s modified method was not different from Tilley and Terry`s organic, matter digestibility of the feedstuffs. In the barley straw organic matter digestibility determined by Tilley`s method the organic matter digestibility of barley straw was proportionatelly increased as the barley straw sample size was increased with slope of (b=38.6 p$lt;0.01). By the Lee`s modified method there was no significant increase (b=36.8) of D.O.M. of barley straw. Linear regression equations of organic matter digestibility of alfalfa and barley-straw were Y=7.28X+52.3 and Y=36.76X+3.01 respectively in the Lee`s modified experiments, where Y=in vivo O.M. digestibility, X=in vitro O.M. digestibility.