Three digestibility trials were conducted using three steers for each; which were with average body weight 392 ± 3.3 kg and 18 month of age. The experimental rations were formulated as follows: ration 1 (R1) 81.8% concentrate feed mixture (CFM) + 18.2% clover hay (CH) (control), ration 2 (R2): 81.55% CFM + 18.05% CH + 0.40% seaweed (S) and ration 3 (R3): 81.23% CFM + 17.87% CH + 0.90% S. The results showed that there were no significant effect of feeding the experimental rations on the digestion coefficients for all nutrients and feeding values, but the digestion coefficients of OM, NFE, ADF, cellulose, ADL, NFC, TDN%, ME (Mcal/kg DM) and NE (Mcal/kg DM) were higher when feeding on R2 or R3 than R1. The predicted values using net carbohydrate and protein system (CNCPS) showed that the sugar and starch fermentation were 276.6 and 729.4 g / d, respectively with feeding R1, 277 and 731.4 g / d, respectively for R2 and 277.6 and 737.2 g/d, respectively for R3. The total microbial protein was 1006, 1008.4 and 1014.8 g/d when feeding on R1, R2 and R3, respectively.    The mean values of ruminal pH, buffering capacity (BC), VFA's were not significant affected by the different dietary treatments, while the concentration of NH₃-N (mg/100 ml RL) tended to increase (p<0.05) when animals were fed on R2 or R3 (13.74 and 14.20 mg/100 ml RL, respectively) than R1 (11.82 mg/100 ml RL). The present study showed that, the dry matter intake and nutrients digestibility were improved by supplemented seaweed but the total VFA's was higher with feeding on 81.55% CFM + 18.05% CH + 0.40 % seaweed (S) than the other rations.