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The ingenious hierarchical structure of enamel composed of rods and protein produces excellent fracture resistance. However, the fracture resistance mechanism in the inner enamel is unknown. The micromechanical models of enamel are constructed to numerically analyze the mechanical behaviors of the inner enamel with different decussation angles and different decussation planes. The results show that the manner of crack propagation in the inner enamel, including crack bridging, crack deflection, and crack bifurcation, is determined by both the rod decussation angle and the decussation plane. In the case of the strong decussation plane, the fracture strength and the required energy dissipation with the decussation angles of 15° and 30° are much higher than those without decussation, demonstrating that decussation is an important mechanism in improving the fracture resistance of enamel. The maximum tensile stress of enamel with the decussation angle of 15° is slightly higher than that of enamel with the decussation angle of 30°, illustrating that an optimal decussation angle exists which balances the strength and toughness. The synergetic mechanism of the decussation angle and the decussation plane on the crack propagation provides a new design hint for bionic composites. [ABSTRACT FROM AUTHOR]