The effect of supplemented seaweed meal from Ascophyllum nodosum (S) to concentrate feed mixture (CFM) fed together with rice straw (RS) on the digestion and fermentation using three lactating Friesian cows, with mean metabolic body size (BW^0.75) of 98 kg was studied. All animals were in the 2^nd to 4^th lactation season. The experimental rations were formulated as follows: R1: ration 1: 69.3 % (CFM) + 30.7% (RS), (as a control ration). R 2: ration 2:  68.3% CFM + 30.7 % (RS) + 1.0% (S). R 3: ration 3:  67.5 % CFM + 31.0 % (RS) + 1.5% (S). These proportions were chosen to achieve approximately iso-nitrogenous and iso-caloric rations.  The dry matter intake (% of body weight) increased when feeding on R2 (4.55) or R3 (4.5) than feeding on R1 (4.13). The apparent digestibility of all nutrients and feeding values were significantly (P<0.05) higher with R1 than the
others, except for nitrogen free extract (NFE) which increased (p<0.05) with R1 and R2 than R3, while unavailable neutral detergent fiber (UNDF) was significantly (P<0.05) higher with R3 than R1 and R2 and there was no significant effect on hemicllulose digestibility. The mean values of the buffering capacity (BC) increased (P<0.05) with R2 than R1 and R3, while the mean values of the total VFA concentration was significantly (P<0.05) higher with R3 than R1, but without significant difference between R1 and R2 or R2 and R3. The mean values for VFA were 7.47, 10.63 and 13.23 ml eq / 100ml with R1, R2 and R3 respectively. The ruminal NH3-N concentrations ranged from 5.5 to 7.13 mg / 100 ml RL with different rations, but without significant differences. The predicted values using net carbohydrate and protein system (CNCPS) showed that the passage rate was higher when feeding on R2 (7.96 %/h) or on R3 (7.11 %/h) than feeding on R1 (6.5 %/h). The fiber, sugars and starch fermentations were higher when feeding on R2 (528, 562 and 2309 g/d, respectively) or with R3 (556, 589 and 2325 g/d, respectively) than feeding on R1 (527, 537 and 2181 g/d, respectively). The total microbial protein was 3245, 3400 and 3471 g/d when feeding on R1, R2 and R3, respectively.  The present study showed that, dry matter intake improved and rumen fermentation, but there were negative effects on the nutrients digestibility. Future, studies are needed to identify the reasons for the decreased nutrients digestibility.