Abstract： Objective　To investigate the effect of placental isoferritin in pathogenesis of preeclampsia and its predictive value. Methods　A prospective double-blinded study was performed. There were 120 initial normal pregnant women at earlier third trimester (from 24 to 34 weeks) in this study. Their plasma placental isoferritin and nitric oxide (NO) metabolites (nitrite/nitrate) (NO2-/NO3-) were examined by enzyme linked immunoabsorbent assay (ELISA) and Griess assay respectively. The outcomes of pregnancies were followed up. Results　In 120 initial normal pregnant women, 19 pregnant women occurred preeclampsia (preeclampsia group), and 101 cases were normal (normal group) at follow up. The levels of plasma placental isoferritin (PLF) were significantly decreased in preeclampsia group (285.31±53.73 mg/L) than that of normal group (699.05±203.03) mg/L, P＜0.01. The levels of plasma NO were significantly increased in preeclampsia group (54.57±32.71 μmol/L) than that of normal group (38.89±30.00) μmol/L, P＜0.05. The significant negative correlation between the plasma placental isoferritin and NO levels was observed（r=0.329，P<0.01）. At the cut point of 400 mg/L PLF level, the sensitivity, specificity, positive predictive value, negative predictive value and Kappa index of PLF level predicting on the outcomes of pregnancy with preeclampsia were 100%, 85.15%, 55.88%, 100%, 0.645. Conclusions　 The decrease of plasma placental isoferritin levels is associated with preeclampsia, and the endothelial cell damage may be one of its mechanism. The PLF may be an earlier predictor of preeclampsia at 24～34 gestational weeks.