Objective: Recent studies have indicated that genetic polymorphisms of the specific genes is one of the causes of miscarriage. In the other hand it is observed that nitric oxide (NO), as a signaling molecule, is participated in primary events of pregnancy such as implantation of blastocyst, differentiation of trophoblast, trophoblast invasion, and it enhances blood supply through the maternal arteries to the placenta. Thus it has a role in implantation and maintenance of pregnancy. NO is produced by endothelial nitric oxide synthase that is expressed in placenta, whereas this enzyme and its production play a key role in early stages of gestation, we investigated relationship between polymorphism of this gene and complication of pregnancy. Materials and Methods: In this study we used the 37 women who had three or more constitutive miscarriage with unknown reason in first trimester of their pregnancy in case group. Control group is included 101 women with normal pregnancy. Both group were analyzed for the VNTR polymorphism in intron4 of endothelial nitric oxide synthase gene by polymerase chain reaction. Results: In patient group genotype frequency of AB is 29.7 %, BB is 64.8 % and AA is 5.5 %. In control group, genotype frequency is 73.2 % for BB, 24.7 % for AB and 1.9 % for AA genotype. Conclusion: The result shows that frequency of AA genotype in the patient group is more than control. Further studies with larger samples need to be done to confirm these findings.