Background: DNA methyltransferase-3B (DNMT3B) is an important enzyme responsible for maintaining the DNA methylation pattern in eukaryotic cells. In this study we have investigated the correlation between the 46359C→T polymorphism in the DNMT3B gene and the risk of breast cancer incidence among sporadic breast cancer patients in Fars Province, Southern Iran.Methods: In this case-control study, 100 breast cancer patients and 138 healthy control subjects were genotyped for the DNMT3B gene by the polymerase chain reaction-restriction fragment length polymorphism method.Results: The genotype frequency in the case (CC 27%, CT 47%, TT 26%) group significantly (P=0.008) differed from the control (CC 19.56%, CT 67.3%, TT 13%) group. We observed a decreased association between the CT genotype and lymph node involvement in breast cancer patients. Our results have shown that in comparison to the homozygous CC genotype carriers the DNMT3B-CT genotype has a significantly lower risk for breast cancer (OR=0.515, 95% CI=0.267-0.994, P=0.048).Conclusion: Our case-control study showed that the CT genotype was significantly associated with decreased breast cancer risk. Consistent with these results, a significant decrease of CT genotype among lymph node positive breast cancer patients was observed. However, a larger study population with more clinical data is needed to confirm these results.