The effect of supplemented seaweed meal as from Ascophyllum nodosum (S) to concentrate feed mixture (CFM) fed together with rice straw (RS) on milk production and composition, some blood constituents, feed efficiency and economic efficiency of lactating Friesian cows was studded. Four lactating Friesian cows were used in a “Swing-over" design, with mean metabolic body size  (BW^0.75 ) of 98 kg. 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 obtained results showed that there was no significant effect of experimental rations on total protein concentration of blood serum, protein concentrations were 7.22, 6.87 and 7.44 g / 100 ml for R1, R2 and R3 respectively. Albumin concentration was highest (P<0.05) with R2 and R3 than feeding on R1, while globulin concentration decreased (P<0.05) with supplemented S. Enzyme activity AST decreased (P<0.05) with R2 than R1 or R3, while ALT decreased (P<0.05) with feeding on R2 than R1. Cholesterol concentration was increased (P<0.05) with R2 than R1.     The urea-N concentration ranged from 18.73 to 26.80 mg/100 ml in the blood-serum, and its levels was significantly (P<0.05) increased when feeding on R2 or R3 than R1. The average glucose concentration ranged from 42.2 to 47.7mg/100 ml blood serum, and its level increased when feeding on R2 or R3 than R1, but without significant effect. The average daily fat corrected milk (FCM, 3.5%) yield was higher with feeding on R1 or R3 (17.36 and 19.15 kg/h/d, respectively) than feeding on R2 (16.55 kg/h/d), but without significant differences. Regarding the milk composition, the total solids (TS) was significantly increased (P<0.05) when feeding on R3 than feeding on R1 or R2. Lactose% significantly increased (P<0.05) when feeding on R3 than R1, while there was no significant effect with feeding on R1 and R2 or R2 and R3. The fat % was higher (P<0.05) when feeding on R1 than R2, but there was no significant effect between R1 and R3 or R2 and R3. The whey protein nitrogen (WPN) or whey protein (WP) concentrations increased (P<0.05) when feeding on R1or R3 than feeding on R2. The feed efficiency was better (P<0.05) with feeding on R3 than R1 and R2, but the economic efficiency was higher (P<0.05) with feeding on R1 than feeding on R2 or R3. So, the price of seaweed must be reduced if the product should have some economical value.