Abstract: Seiberg Duality conjecture is proposed by Yongbin Ruan and relates GLSM of two different quivers related via quiver mutation. In this talk, I will introduce the result of the conjecture applied to $A_n$ type quiver and we prove that small I functions of flag variety (before duality) and a complete intersection in another quiver (after mutation) are equal under variable change. Moreover, we can extend our result to the tautological bundle over flag variety.