In the present work, a duct geometry is optimized to reduce the flow distortion using adjoint method. The S-duct geometry is from AIAA Propulsion and Aerodynamics Workshop (PAW [1]). ANSYS Fluent Adjoint solver is used to run the design optimization study. The workflow involves a 4-step process: CFD run, adjoint calculation, design calculation based on the sensitivity data and environment constraints, and mesh update. This process is repeated until the optimum geometry is obtained. The variance of total pressure at the Aerodynamic Interface Plane (AIP) is chosen as the quantity of interest. It correlates well with the circumferential and radial distortion index (CDI & RDI) as defined in [2], which indicates flow distortion at the inlet/exit plane. After 3 design iterations, the variance of total pressure and the average CDI at AIP was reduced by 20.91% and 8.36% respectively, while the maximum CDI and RDI showed a drop of 2.15% and 12.37% respectively.