Studies in model plants showed that SEPALLATA (SEP) genes are required for the identification of floral organs and the determination of floral meristems in Arabldopsls. In this paper a SEP homolog, TrSEP3, was Isolated from a SEP3-clade of SEP (prevlous AGL2) subfamily. In sltu hybridization was used to reveal the potential functional specification, and the results showed that TrSEP3 expression was first observed in floral meristems and then confined to the floral primordia of the three inner whorls. In the matured flower, TrSEP3 was strongly expressed in the tips of pistils and weak in stamens and petals. The evolution force analysis shows that TrSEP3 might undergo a relaxed negative selection. These results suggested that TrSEP3 may not only function in determining the identity of floral meristems and the primordia of three inner whorls, but also function in matured reproductive organs.