Medicinal plants have been used for traditional treatment and known to possess antioxidant activities that may help to reduce the risk of oxidative stress-related diseases. Chemically-induced oxidative hemolysis is usually associated with the formation of Heinz bodies-insoluble precipitates within red blood cells, indicating the oxidative destruction of hemoglobin. In the present study, antioxidant activity was investigated by Heinz body induction in vitro model for providing preliminary data selection of plant extracts compared to tannin concentration in Thai mimosaceous plant extracts. Twenty-one Thai mimosaceous plants, extracted by ethanol, were screened for tannin concentration and antioxidant activity of inhibitory Heinz body induction under in vitro condition. The study showed that the percentage of Heinz body inhibition activity from Parkia speciosa seed coat was the highest and followed by Xylia xylocarpa bark, P. speciosa pericarp, and Entada rheedii seed coat, respectively with inhibition concentration 25 (IC 25) of 2.68, 15.71, and 28.14 mg/ml, respectively. In addition, X. xylocarpa bark, E. rheedii seed coat and P. speciosa Hassk. seed coat also contained high tannin concentration. The percentage of Heinz body inhibition activity from plant extracts at dilution 1: 2 and 1: 10 (50.00 and 10.00 mg/ml) were shown correlation with tannin concentration (p < 0.01, r = 0.658 and p < 0.05, r = 0.536, respectively). This study also found that tannin could be found in all plant extracts and also inhibited the Heinz body induction at the minimum level of 0.625 mg/ml. Mimosaceous plants contained antioxidant activity demonstrated by Heinz body inhibition activity and most of the plant extracts with high Heinz body inhibition activities might be caused by high tannin concentration. This data would be valuable sources for searching potent medicine in the future.