In the present study, we examined the association of different alleles of MICA gene with the risk of breast cancer development in Iranian population. Our data showed a significant relationship between longer alleles, alleles with 9- and 6-GCT repeat of MICA gene, and a higher risk of developing breast cancer according to the age of onset. The data indicated a 6-fold increase for developing breast cancer in patients carrying the allele with 6-GCT repeat after age 50 (OR = 5.8333, 95% CI: 1.2976-26/2236, P = 0.0172). In addition, patients carrying longer alleles in their genotype (6/6, 6/9, and 9/9 genotypes) were found significantly at higher risk of developing breast cancer than control individuals (OR = 5.6, P = 0.0038, 95% CI: 1.6578-18.9166). In contrast, alleles with short GCT repeat of 4 and 5.1 showed to play a role in reducing the risk of breast cancer (OR = 0.79, P = 0.3643 and 95% CI: 0.4743-1.3157). Women with allele 4 were found twofold more protected against breast cancer (OR = 0.4597, 95% CI: 0.2164-0.9765, P = 0.0401). The results suggested that women with genotypes with 9- and 6-GCT repeat alleles of MICA gene could be considered more potent to develop breast cancer especially at higher age.