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In this paper, we demonstrate for the first time how partial/full replacement of Co (metallic phase) with ZrO2 (ceramic phase) in WC–Co system, along with spark plasma sintering, can lead to the development of high-performance WC-based ceramic nanocomposites. Convergent beam electron diffraction analysis, in combination with transmission electron microscopy, reveal the dispersion of nanosized t-ZrO2 particles, as well as the Co phase, in the dense WC–ZrO2–Co nanocomposites. In order to obtain reliable measures of strength and fracture toughness properties, four-point bending configuration, and single-edge V-notch beam techniques, respectively, were used. Among the investigated nanocomposites, WC–6 wt% ZrO2 inter/intragranular nanocomposite exhibited the optimum combination of mechanical properties such as high hardness (∼20 GPa), flexural strength (∼1.3 GPa), and fracture toughness (∼10 MPa·m1/2). The flexural strength was superior by ∼18% to that measured with the reference WC–6 wt % Co cermet, while the fracture toughness was only modestly lower by ∼16%. Theoretical estimates, based on residual stress-induced toughening, were found to be insignificant to explain the high toughness of the nanocomposites. The ability to maintain considerably high fracture toughness has been attributed partly to the transformation toughening by the t-ZrO2 phase. Additional contributions from other toughening mechanisms originate from the change in fracture mode from intergranular (WC–6 wt% Co cermet) to transgranular in the presence of nanosized ZrO2 particles, crack bridging, and crack deflection by the ZrO2 particles. Based on finer scale microstructural analysis as well as mechanical property measurement, an effort has been made to establish the structure–property relationship in the investigated ceramic nanocomposite system.