A series of Ni/C catalysts with different Ni content (15, 20, and 30 wt% Ni) were prepared by the wet incipient impregnation method. Their textural properties were studied by surface fractal dimension (D
s) and nonlocal density functional theory using nitrogen sorption data. Their structural properties were studied by x-ray diffraction, Rietveld refinement, radial distribution functions (RDFs), and electron density maps of Fourier. Surface areas of Ni/C catalysts decreases slightly from 614 to 533 m2/g as Ni content increases from 15 to 30 wt%; however, the Ni crystallite size (5.1–31.4 nm) increases as the nickel content increases. Many point defects were found by Rietveld refinement in nickel nanostructures of Ni/C catalysts with 20 and 30 wt% Ni. This was confirmed by RDFs and electronic density maps. On the other hand, the hydrogen production via the photodehydrogenation of ethanol is very sensitive to the nickel crystallite size and the number Ni atoms in nickel nanostructures. The maximum reaction rate (363.64 μmol/h) is achieved on Ni/C catalyst with 15 Wt% Ni content which has the smallest crystallite size (5.1 nm) and less point defects in its nickel nanostructures. Ab initio calculations were performed to propose a reaction mechanism in the photodehydrogenation of ethanol.