Natural nacreous composites such as nacre, teeth and bone have long been extolled for their higher strength and toughness. Understanding the toughening and strengthening mechanisms as well as the condition triggering their occurrence would be of great value to the biomimetic synthesis. In this paper, our attention is mainly focused on crack deflection and flaw tolerance, which were reported as crucial toughening and strengthening mechanisms in nacreous biological materials, respectively. By applying the "brick-and-mortar" (B-and-M) structure model, our finite element-based simulation showed that the propagating direction of a crack ending at the brick/mortar interface could be controlled by tuning the fracture strength of brick. Subsequent examination on the tensile strength (TS) of the cracked B-and-M structure indicated that in nacreous composite flaw tolerance can be achieved below a length scale determined by the ductility of mortar phase. These findings would serve as guidelines in the design and synthesis of novel biomimetic materials aiming at higher strength and toughness. ; Department of Mechanical Engineering