This study aimed to evaluate the effects of adding feed additives such as sodium bicarbonate (SB), magnesium oxide (MgO), calcareous marine algae (CMA), or seaweed to calves' rations on performance, rumen fermentation, blood parameters, and the economic efficiency of Holstein's calves. Twelve healthy Holstein calves aged nine months with an average body weight of 272.7 ± 3 kg were randomly allocated into four dietary treatment groups. The animals in all groups received a basal diet consisting of a concentrate feed mixture (CFM) and wheat straw. The calves in the 1^st group (T1) were fed CFM supplemented with 1% SB, the 2^nd calves' group (T2) were fed CFM supplemented with 1% SB and 0.3% MgO, the 3^rd calves' group (T3) were fed CFM supplemented with 1% CMA, and the 4^th calves' group (T4) were fed CFM supplemented with 1% CMA and 0.3% MgO. The results revealed that the average daily gain, DMI, and feed conversion ratio were not significantly affected by dietary treatment. The calves in T3 had a higher (P<0.05) plasma albumin and A/G ratio and a lower (P<0.05) globulin compared with the other groups. The value of serum glucose was higher (P<0.05) in T1 than the other groups, with no significant differences observed among the T2, T3, and T4 groups. The values of ALT were increased in T2 and T3 compared with the T1 and T4 groups. Supplementation of antacids to calves' diets did not have a significant effect on ruminal pH or ammonia nitrogen concentration. The ruminal TVFAs increased (P<0.05) with the T2 diet compared with T1 and T4. In conclusion, dietary sodium bicarbonate and seaweed supplementation in calves' diets improve some blood parameters and rumen fermentation, particularly when mixed with MgO.