Ni-doped ZnO (Zn1-xNixO, with 0.25 <= x <= 0.50) diluted magnetic semiconductors were prepared by the solid state reaction method. We have studied the structural properties of the samples by using the X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) and Energy Dispersive X-Ray spectroscopy (EDX) techniques. The SEM and AFM results clearly demonstrate that Ni2+ ions integrate into the ZnO structure without any problem. The grains of the samples are very well connected to each other and tightly packed, and vary in size from 0.2 mu m to 2 mu m. From the XRD and EDX spectra of the samples, it has been concluded that the doping causes no change in the hexagonal wurtzite structure of ZnO. However, the XRD indicated that three additional peaks, related to the (102),(012) and (108) planes appeared for the doped samples. Furthermore, an additional NiO-associated diffraction peak appears for the highest concentration of Ni, i.e., for x=0.50 of Ni2+ doping, which indicates an upper limit for Ni concentration. The estimated crystal sizes from the XRD results vary from 4.38 A to 9.73 angstrom. The lattice parameter a and c of Ni-doped ZnO are slightly smaller and higher than that of pure ZnO, respectively. These observations may be due to the slightly different ionic sizes of Zn2+ and Ni ions.