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This experiment was conducted to determine digestibility of dry matter (D.M4), Neutral-detergent fiber (N. D . F.) Acid-detergent fiber (A .D .F.) . Cellulose (C) Hemicellulose (HC.) , Lignin (L.) , Nitrogen (N.) and Energy (E) of three sheep rations by in vivo , in vitro rumen fermentation and chemical procedure (summative equation ) of van soest . The rations were hay ( late cut rye, 80-85 % and clover , l5-2o) , pellets and hay plus pelleted concentrates ( in equal portion ) and were designated as 1, 2 and 3 , respectively . in vivQ digestibility of D.M., N.D.F., A.C.D., C., HC •, L., N., and E. of the rations were measured, using nine mature corriedale wethers, each fitted with a permanent rumen cannula in conventional trials . The sheep were divided into 3 groups ( three for eacl2 group ) which were fed ration 1,2 and 3 at maintenance level for 11 months. In the same time, in vitro digestibility of these nutrient contents of the rations was determined . Also dry matter digestibility of the rations was predicted by summative equation . The summative equation gave mean values 60.7+2.5, 74.8 , 2.1 and 65.7 • 1.8 for the ration 1,2 and 3 , respectively . While t: in viva gave values that were 2.8 , 3.7 and 2.5 Units smaller for the three rations, in the same order . The mean values in vitro digestibility of D.M. were 60.9+2.9, 77.8±1.2 and 68.8±2.7 for the ration 1 , 2 and 3 respectively . The mean values in vivo , digestibility of N.D.F. were different fran in vitro values, but the difference was not significant . The mean values in vivo lignin digestibility in rationi1 2and 3 were almost two times greater than the mean values in vitro rumenfermentatioj • The variation between these two methods was highly significant (P< 0.01 ). Ther. was a significant ( P< 0.01 ) difference between the mean values 4 viva and vitro rumenfermentation digestibility of A.D.F. far ration 2. Also this case was true far cellulose