A recent genome-wide association study identified a novel single nucleotide polymorphism (SNP), rs2046210, in the 6q25 region as a breast cancer susceptibility locus in Chinese and subsequently replicated in a multicenter study. Further fine-mapping of this region may help identify the potential causative SNPs of breast cancer. We employed a block-based fine mapping analysis to investigate the tagging SNPs in a 41 kb block with the marker-SNP rs2046210 in the 6q25 region, and also extended our study by including two potentially functional SNPs (rs2234693 and rs1801132) within the ESR1 gene by a two-stage case-control study with 1,792 breast cancer cases and 1,867 controls (878 cases and 900 controls in the testing set and 914 cases and 967 controls in the validation set). Significant associations with breast cancer risk were observed for rs1038304, rs6929137, rs2046210, and rs10484919 in the 41 kb block of the 6q25 region in the testing set after controlling multiple testing. Together with the validation set samples, these four SNPs were all significantly associated with increased risk of breast cancer (additive OR from 1.25 to 1.34, additive P from 4.84 × 10(-6) to 7.17 × 10(-9)). After conditional regression and linkage disequilibrium analyses, rs6929137 and rs10484919 tend to be susceptible markers of breast cancer in this region and both of them were located at sites of histone modification according to the UCSC (http://genome.ucsc.edu/) genome database. Our results support that the 6q25 region is an important susceptibility region for breast cancer in Chinese women, and rs6929137 and rs10484919 are causative or marker SNPs for this region.