Background: A new biological dosimeter based on methemoglobin level was developed in this study. Materials and Methods: Methemoglobin level in erythrocytes from mice subjected to &gamma rays from a ⁶⁰Co source was detected using the methemoglobin kit. The dose range was from 0.5 to 8 Gy and the dose rate was 0.5 Gy/min. Results: The results demonstrate that methemoglobin level increases with increasing dose. The detection limit based on methemoglobin has a lower limit of dose estimation of about 1 Gy. The high levels of methemoglobin are maintained for at least 28 days, and the maximal increase of methemoglobin observed occurs at about 30 min after &gamma irradiation. The relationship between dosage and the increased methemoglobin level can be expressed by a linear quadratic equation of y = -8.75 x2 + 168.09 x + 32.66, with the correlation coefficient, r, equal to 0.96. The best suggested time for blood collection is up to 1 day after &gamma irradiation. The doses absorbed by mice as estimated from the use of the dose-response relationship were close to the blind doses of 1, 2, 4 and 8 Gy. Conclusion: Methemoglobin is a quick, simple, and precise biomarker for the early assessment of the absorbed dose in mice.