To study the forage utilizability by young lambs, Romney lambs were weaned ar 3.5 weeks, 5.5 weeks, 7.5 weeks and 9.5 weeks of age and put onto alfalfa and perennial rye grass pasture or metabolic crates for two or four weeks and estimated in vivo organic matter digestibility. These lambs were slaughtered and collected the rumen liquor which was used for in vitro fermentation of alfalfa and perennial rye grass. There was no difference between in vitro and in vivo organic matter digestibility. However, there was a significant difference (P$lt;0.05) between the treatment of 5.5 week-slaughtered (3.5 week-weaned) and 9.5 week-slaughtered (7.5 week-weaned) and between 9.5 week-slaughtered (7.5 week-weaned) and 11.5 week-slaughtered (9.5 week-weaned) at 5% level of probability. In the in vitro and in vivo organic matter digestibility by the lambs slaughtered at 9.5 weeks of age, there was no significant difference in digestibility of organic matter of alfalfa and perennial rye grass. However, a significant (P$lt;0.05) difference with sample cut on November 3rd was observed. There was no over all feed sample difference, however, there was difference between 9.5 weeks and 11.5 week-slaughtered group. And there was also a significant difference between feed sample cut-on September 28th, October 19th and November 3rd in organic matter digestibility. A high correlation coefficient (r=0.94) was observed between the over all in vitro and in vivo organic matter digestibility. A significant correlation coefficient (r=0.88) was found between in vitro and in vivo organic matter digestibility of alfalfa. The linear equation, D.O.M. _(in vivo)=0.86 D.O.M. _(in vitro)+12.1 was estimated from these results. For perennial rye grass there was a high, correlation (r=0.99) between in vitro and in vivo organic matter digestibility and calculated linear regression equation, D.O.M. _(in vivo)=1.105 D.O.M. _(in vitro)-5.95.