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Abstract Introduction: Free radicals generated by ionizing radiations, as a powerful cytotoxic agent, can damage DNA and proteins. Thymus vulgaris L (thyme) plant is a rich source of antioxidant phenolic compounds, which makes it a preferable candidate for medical applications. Given this, we set out the present study to investigate the effectiveness of thyme essential oil on human blood mononuclear cells (PBMCs) as a radioprotector agent against ionizing radiations. Materials and Methods: We extracted the thyme essential oil by the conventional Clevenger extraction method. Heparinized peripheral blood samples were also collected from five male volunteers, aged 22-25, without a history of smoking and irradiation. PBMCs were isolated and the maximum nontoxic concentrations (85µg/ml (of thyme essential oil were determined based on the result of the MTT method. In the next step, the PBMCs were cultured in the presence of thyme essential oil before and after X-irradiation with doses of 0.25 and 2.00 Gy. Results: The most radioprotective effect was observed in the dose of 2.00 Gy for thyme-treated cells 24 hours before the irradiation (p-value≤0.001) by a survival enhancement factor of 1.67, compared to the control group. Conclusion: Our results showed that thyme essential oil can be used as an effective radioprotector agent for PBMCs against ionizing radiations. The most radioprotective effect was observed in the presence of thyme essential oil during irradiation.