OBJECTIVE To determine the relationship of selenium plasma level and preeclampsia, as an antioxidant, against reduction damage of free radicals. METHODS A case control study was carried out on 40 preeclamptic, and 40 healthy pregnant women in 34-39(th) week of gestation who were matched on the age of pregnancy and mineral supplements intake, in Iran. The plasma level of selenium measured by atomic absorption. Data was analyzed by using the statistical tests (t-test, chi-square, regression), α = 0 .05. RESULTS The mean plasma level of selenium significantly decreased in preeclamptic women compared to healthy pregnant women (p = 0.001). There was a relationship between plasma levels of selenium and preeclampsia. The odds of having preeclampsia in women who were at the bottom tertile of plasma selenium was 4.96-fold (OR = 4.96, CI 95%; 1.56-15.6, P = 0.006) and in the second tertile 3.94-fold greater (OR = 3.94, CI 95%; 1.26-12.33, P = 0.019) than third tertile of plasma selenium. Selenium plasma level under 60 μg/L has a sensitivity of 87.5%, a positive predictive value of 64.8% and Youden index was 0.4. CONCLUSION The result of this study shows that the reduction of selenium plasma level in preeclamptic women in comparison with healthy pregnant women may demonstrate a higher demand of Se in preeclamptic pregnancies. Selenium plasma level <60 μg/l might be a favorite cut-off point for screening of preeclampsia specially in pregnancies with low selenium plasma level; however, it may not be a favorite predictive.