To provide human needs, it is very important to produce more milk and meat from less number of animals. So improvement in reproduction will lead to produce more milk in every one day of animal?s longevity. To reach this purpose, genetics and animal breeding are very essential. There are a lot of genes affecting quantitative traits, so direct selection of these traits would be very difficult and complicated. Finding a genetic marker which physiologically affects the quantitative traits? expression or is in linkage with major genes with great influences on reproductive traits, may improve them, by increasing selection accuracy and decreasing generation interval. Estrogen is one of the major hormones that have a large effect on growth, differentiation and function of many tissues such as reproductive tissues (uterine, ovary, oviduct, mammary glands, testis and prostate). So because of the estrogen?s main role in regulation of reproduction, it?s alpha subtype receptor gene ( ER? ) have been used as a genetic marker for reproductive traits. ER? protein is distributed in many tissues of male and female reproductive organs and has a key role in regulation of reproduction physiology. In this study, ER? gene polymorphism in 5? region, in consensus promoter for exon C, A/G transition, and it?s relation with reproductive and productive traits and birth weight was investigated. Genomic DNA was isolated from 200 dairy Holstein in four farms in Isfahan province, by using Miller salting out DNA extraction procedure. Then they were genotyped by PCR-RFLP technique, as the DNA fragments containing the mutation were amplified using special primers and then digested with BglI endonuclease enzyme. Fragments with 245 bp represented allele A and those with 168 bp and 77 bp were equivalent to allele G. Allelic frequencies were 0.0742 and 0.9257 respectively and the distribution of AA, AG and GG genotypes was 0.010, 0.129 and 0.861 respectively. According to the calculations, the studying population was in Hardy-Weinberg equilibrium. It means that this gene probably have not been under control of equilibrium disturbing factors such as selection, mutation, migration and genetic drift. It is probably because of the fact that ER? gene has not been selected in animal breeding purposes. Using SAS software (Proc GLM), the effect of ER? gene polymorphism on reproductive and productive traits and birth weight was investigated. Herd and genotype were considered as fixed effects and milking days, dry days and open days, as covariates. Results showed that herd had significant effects on gestation length, birth weight, crude milk and fat and protein production traits, but Key Words: estrogen receptor alpha, polymorphism, genetic marker, reproductive traits, A/G transitio