The morphology of the copper iodide (CuI) film as an inorganic p-type material has an important infl uence on enhanc-ing the performance of polymer solar cells (PSCs). A self-assembled monolayer of 3-aminopropanoic acid (C3-SAM) was used on the surface of indium tin oxide (ITO) before depositing the CuI films. Consequently, a well-distributed and smooth CuI film was formed with pinhole free and complete surface coverage. The root mean square of the corresponding CuI film was reduced from 3.63 nm for ITO/CuI to 0.77 nm. As a result, the average power conversion efficiency (PCE) of PSCs with the device structure of ITO/C3-SAM/CuI/P3HT:PC61BM/ZnO/Al increased significantly from 2.55％(best 2.66％) to 3.04％(best 3.20％) after C3-SAM treatment. This work provides an effective strategy to control the morphology of CuI films through interfacial modification and promotes its application in efficient PSCs.