This paper reported the synthesis, crystal structure and electrical conductivity properties of Ni-doped ZnO powders (i.e. Zn1-XNiXO binary system, X=0, 0.0025, 0.005, 0.0075 and in the range 0.01 <= X <= 0.15). I-phase samples, which were indexed as single phase with a hexagonal (wurtzite) structure in the Zn1-XNiXO binary system, were determined by X-ray diffraction (XRD). The widest range of the I-phase was determined as 0 <= X <= 0.03 at 1200 degrees C; above this range the mixed phase was observed. The impurity phase was determined as NiO when compared with standard XRD data, using the PDF program. We focused on single I-phase ZnO samples which were synthesized at 1200 degrees C because of the widest range of solubility limit at this temperature. It was observed that the lattice parameters a and c of the I-phase decreased with Ni doping concentration. The morphology of the I-phase samples was analyzed with a scanning electron microscope. The electrical conductivity of the pure ZnO and single I-phase samples were studied by using the four-probe dc method at temperatures between 100 and 950 degrees C in air atmosphere. The electrical conductivity values of pure ZnO and 3 mol% Ni-doped ZnO samples at 100 degrees C were 2x10(-6) and 4.8x10(-6) Omega(-1).cm(-1), and at 950 degrees C they were 1.8 and 3.6 Omega(-1).cm(-1), respectively. In other words, electrical conductivity increased with Ni doping concentration.