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We report the structure and magnetic properties of Zn1-xMnxO thin films grown on Si(001) substrates by pulsed laser deposition. Structure and phase evolution with Mn doping has been studied using x-ray diffraction, electron diffraction, and high-resolution electron microscopy. The undoped and 1% Mn-doped ZnO films are completely (001) oriented, and further Mn doping deteriorates the (001) orientation. For Mn concentrations below 3%, only the hexagonal ZnO phase exists in the films without secondary phases. As the Mn concentration reaches 5%, secondary phase Mn2O3 was found aggregating at grain boundaries. All the Mn-doped films show ferromagnetic properties at room temperature, and the magnetic moment decreases as the Mn concentration increases. Our results suggest that the ferromagnetism observed in Zn1-xMnxO thin films is intrinsic rather than associated with secondary phases.