Novel nanostructures of self-assembled Co-doped ZnO films were successfully synthesized by hydrothermal synthesis process. Our investigation indicates that the growth reaction rate is the key factor that controls the morphology evolution and crystallographic orientation of self-assembled Co-doped ZnO films via changing the reaction solution concentration. The preferential growth direction of Co-doped ZnO films alters from polar [ 0001] to non-polar [10-10] lattice direction by increasing the concentration, which facilitates the transformation from 1D nanorods, assembled in straw-like structure, to 2D nanosheets with dendrite-like structure. Interestingly, room-temperature ferromagnetism is observed in both of polar (0001) c-plane and non-polar (10-10) m-plane Co-doped ZnO films. As the most promising nanostructures, the assembled nanostructured Co-doped ZnO films may find future applications in spintronics devices. (c) 2017 Elsevier B.V. All rights reserved.