Objective: Down syndrome is the most common cause of mental retardation observed approximately 1 in 800 live births. The cause of Down syndrome is chromosome 21 trisomy has been shown to be maternal in 86% of cases. Common polymorphisms in folate genes has been suggested as maternal risk of chromosomal nondisjunction in some population. Materials and Methods: In present study, we investigated the association of combined CBS 844ins68 and MTRR A66G polymorphisms in folic acis pathway among 65 Iranian mothers having Down syndrome children and 85 aged matched control mothers. Results: Statistical analysis showed that frequency of ins+ allele in CBS and G allele in MTRR were higher in control mother than case group. Combined CBS ins+/- genotype with MTRR AG genotype was significantly was higher in control mothers (p value=0.039) but MTRR GG genotype together with CBS ins+/- was not significant (p value > 0.05). Conclusion: Our data suggest GG genotype maybe lethal and CBS 844Ins68, MTRR A66G provided protective association for genotype Ins+/-, AG. This information is important for better understanding of etiology of Down syndrome.