Abstract: The oxygen radical absorbance capacity (ORAC) assay method has been employed extensively in the field of antioxidant and oxidative stress. The ignorance of the effect of salt ion in the sample on ORAC method resulted in the incorrect evaluation of the antioxidant capacity. The present study investigated the effect of salt on ORAC assay’s accuracy in terms of adding different salt (including NH 4 Cl, CH 3 COONa, CH 3 COONH 4 and NaCl) to Trolox, ORAC values increased due to the the salt ions in the sample, especially CH 3 COONa, which may be attributed to the fact that the change of ionization of fluorescein caused by salt ion lead to the alteration of its oxidation rate. Therefore, measuring error could be eliminated by addition of same concentration of salt ion in tested sample in Trolox for determination of standard curve.