The effective fracture toughness (EFT) of ZrB2-C ceramics with different engineered microarchitectures was numerically evaluated by phase-field modeling. To verify the model, fibrous monoliths (elongated hexagonal ZrB2-rich cells in a continuous C-rich matrix) with different volume fractions of a C-rich phase were considered. Architectures containing 10 and 30 vol% of C-rich phase showed EFT values about 42% more than that of pure ZrB2. Increasing the C-rich phase to 50 vol%, dropped toughness significantly, which is in agreement with the experimental results. Replacing hexagonal cells with cylindrical, triangular, or square cells of the same cross-sectional area changed the toughening mechanism and EFT. The orientation of the interface between the soft and hard phases with respect to the crack orientation also affected the energy required for crack propagation, and in some cases resulted in a higher EFT (even up to 70% of pure ZrB2 fracture toughness) either by suppressing uniform crack propagation or making crack cranking. Results not only show that the model can predict fracture toughness but also provide insight to improve toughness by engineering different microarchitectures.