Foxn1 is essential for thymus development.The relationship between boron and thymus development, optimal dose of boric acid in ostrich diets, and effects of boron on the expression of Foxn1 were observed by the histological changes of thymus and investigated by the expression of Foxn1 in thymus.Ostrich Foxn1 was sequenced by Race method.Foxn1-positive cells were mainly distributed in thymic medulla, rarely in cortex.With the increasing of boric acid concentration, Foxn1 expression was dynamically increased in thymus, mean density (MD) of Foxn1-positive cells was the highest at 80mg/L (group Ⅲ).The MD of Foxn1-positive cells was decreased at above 320mg/L, and at 640 mg/L, it could significantly inhibit Foxn1 expression and thymus development.The nucleotide sequence and the encoded protein of Foxn1 were 2736 bases and 654 amino acids in length.It is highly conserved as compared with other species.These observations demonstrated that low boron concentration played a protective role in the thymus development, while high boron concentration could damage the organ or even produced toxic effect.The optimal dose of boric acid in ostrich diets is 80 mg/L drinking water and these findings are a substantial advancement over current understanding of thymus response to boron stress.