This study investigated the effect of different concentrations of humic acids (HAs) on the selected serum biochemistry parameters and oxidative status markers in common pheasants (Phasianus colchicus). The control birds were fed a diet with no HA additives, while the birds in the experimental groups were fed diets containing HAs at the level of 0.5% (EG1), 0.75% (EG2) and 1.0% (EG3) from 1 to 90 days of age. The blood sodium concentration decreased (P < 0.01) in the group fed by 1.00% HAs compared to the control birds. The concentration of potassium in the EG2 group increased (P < 0.05) in comparison to the control group. The EG1 group showed a higher (P < 0.01) serum glucose than the EG2 group. Significant differences (P < 0.05; P < 0.01) were also found between the experimental groups (EG1 vs EG2, and EG2 vs EG3) in the cholesterol concentrations. The birds in the experimental groups showed lower ROS (reactive oxygen species) and MDA (malondialdehyde) production. An opposite effect was observed in the TAC (total antioxidant capacity), where its values significantly increased in the experimental groups. The diet supplementation affected the enzymatic antioxidant system of the fattened pheasants, and so the HAs exhibited an antioxidant potential in these birds.