Shape-controlled fabrication of non-noble metal nanoparticle (NP) is still challenging. Herein, we report confined hexahedral nickel NP catalyst facilely prepared by reducing tartrate-intercalated Ni2+, Al3+-containing layered double hydroxide (NiAl-LDH) precursor in flowing N-2 at controlled reduction temperature. Spherical aberration-corrected scanning transmission electron microscopy, combined with other characterization techniques, was used to identify morphology and structure of the resulting Ni NP catalysts and to reveal the formation process of the confined hexahedral Ni NPs. In situ formed amorphous carbon between LDH interlayers can induce explosive formation of abundant Ni nuclei during the N-2 heat treatment. Further growth of Ni nuclei by reducing Al3+-doped NiO [i.e., Ni(Al)O-x oxide] results in the metastable hexahedral Ni NPs inserted in mixed domains of Ni(Al)O-x weakly crystalline partitioned by amorphous carbon. In addition, coalescence of Ni NPs at elevated temperature leads to sphere-like Ni droplets with a larger average size. The hexahedral Ni NP catalyst shows higher catalytic activity than the sphere-like counterparts in reduction in p-nitrophenol to p-aminophenol. The hexahedral shape feature of Ni NPs, as well as the synergistic effect between Ni NPs and their surrounding mixed domains, was proposed to link with the enhanced catalytic performance.