This experiment was conducted to determine the digestibilities of forage crops with the method of in vitro fermentation technique and to estimate the in viva digestibilities from the in vitro results. The results obtained are as follows: 1. The result of the present experiment gave no significant difference in the in vitro dry matter (D. M.) digestibilities of the standard grass when inocula from 3 fistulated wethers on the same feed were compared. 2. No significant difference was represented between the in vitro digestibilities of the standard feed samples with rumen inocula collected at 1, 5, and 15 hours after feeding. 3. The in vitro digestibilities tended to increase with longer first stage fermentation periods, but no significant difference was found. 4. No significant difference was represented between the amount of substrate. 5. Analysis of variance of in vitro D.M. digestibilities of standard grass showed that there was no significant difference between days. 6. The in vivo digestibility was estimated from in vitro one with the regression equation and the standard error of estimate, Y=0.708×+18.37 and 1.24, respectively. 7. With the advance of fall season, the in vitro digestibilities tended to decrease, however, and the correlation coefficients with crude potein and crude fiber content were not significant.