Sex is the most important exchange method of the genetic information in eukaryotes. Sex determination mechanism is varied depending on the species, however, sex differentiation mechanism in vertebrates are basically similar. Sex determination of birds is controlled by the sex chromosomes and their sexual differentiationare critically controlled by estrogen. Because estrogen is biosynthesized by aromatase, it is possible to find the sex-determining gene of birds by analyzing the transcriptional regulatory mechanism. While there have been conducting research on the basis of this concept, the mechanism has not yet been fully elucidated. On the other hand, DMRT1 gene has been cloned as a strong candidate in the sex-determining gene of birds. Analysis of cascade leading to the aromatase gene from DMRT1 gene is an important issue in the future.