Developing metal oxide nanocrystals with tailored crystal facets is of great importance for heterogeneous catalysis. Herein, we synthesized three kinds of Co3O4 nanocrystals with exposed different facets by hydrothermal method and explored the facet-dependent reactivity in soot combustion for the first time. The structural, morphological and chemical properties of the obtained Co3O4 nanocrystals have been systematically analyzed and well correlated with their soot combustion efficiencies. The results indicated that their catalytic activities ranked in the order of Co3O4 nanocubes with exposed (001) facets > Co3O4 truncated octahedrons with co-exposed (001) and (111) facets > Co3O4 octahedrons enclosed by (111) facets. This superior activity of Co3O4 nanocubes was originated from the chemical nature of the exposed (001) facets, which contained more Co3+ cations, surface defective sites, higher surface lattice oxygen mobility and excellent redox properties, thereby improving the overall soot combustion efficiency. Meanwhile, the isothermal kinetic study also provided direct evidence to validate the crystal facets rather than the surface area to play the critical role in determining the soot combustion efficiency. Furthermore, the Co3O4 nanocubes exhibited strong stability against structural collapse owning to the well-defined shape and stable crystal phase, showing its potential applications in real practical conditions.