ntroduction. Preelampsia is a common and serious complication of pregnancy that affects both mother and newborn. This study designed to determine whether low-dose aspirin or calcium supplements taken throughout pregnancy reduce the incidence of preeclampsia. Methods. The present study was a randomized controlled clinical trial. 990 healthy nulliparous women, who were pregnant for 13 to 20 weeks were chosen to receive daily treatment with low dose of aspirin 75 mg (330 patients, aspirin group), calcium D 500 mg (330 Patients, calcium group) and no medication (330 patients, control group) for remainder of theirs pregnancies. Data included demographic, obstetric, prenatal care, hospital records and final diagnosis were collected. Statistical analysis was performed using Chi-Squre, ANOVA and Duncan test at significance level of 0.05. Results. Preeclampsia occurred in 15 of 330 women in the aspirin group (4.6%), 13 of 330 women in the calcium group (4%) and in 33 of 330 women in control group (10.1%). There were significant differences between aspirin and control groups (4.6% vs. 10.1% P < 0.05) also between calcium and control group (4% vs. 10.1% P < 0.05). Discussion. These results suggest that low dose of aspirin or calcium D during pregnancy in healthy nulliparous women is effective to reduce the prevalence of preeclampsia.