Background: PROM is a complication that affects 10% to 20% of all pregnancies. The participation of vitamin C in the development of PROM has been suggested owing to its well described role in the biosynthesis of collagen, the main structural constituent of chorioamniotic membrane, this hypothesis has been partially supported by the finding of low mechanical resistance and decreased collagen concentration in samples of fetal membranes that ruptured prematurely. Aim of work: The present study was performed to study the effectiveness of 100 mg vitamin C in preventing PROM. Patient and methods: A controlled single-blind trial was performed.126 pregnant women in their 20th wk of gestation were invited to participate; 109 women accepted add were randomly assigned to 2 groups (100 mg vitamin C or placebo), antenatal visit every 4 wk was done ,and subject was evaluated for cervicovaginal infection. The incidence of PROM was recorded for each group as an indicator of the protective effect of vitamin C supplementation. Results: the incidence of PROM was 4 (8%) of 52 women in the vitamin C supplemented group and 14 (25%) of 57 women in placebo group (P=0.018). The incidence of preterm labour was 21 (19%) in 109, but there was no significant difference between groups (24% in the placebo group and 13% in the vitamin C group; p=0.142). preterm labor occurred in 9 of 18 pregnancies complicated with PROM and in 21 of 91 pregnancies not complicated with PROM (RR: 6.58; CI: 2.17, 19.89) Conclusions :our data supports the concept that vitamin C at dietary doses (100 mg /day) can prevent PROM .