Biological nanocomposites, such as bone, tooth, shell, and wood, exhibit exceptional mechanical properties. Much recent effort has been directed at exploring the basic mechanical principles behind the microstructures of these natural materials to provide guidelines for the development of novel man-made nanocomposites. This article reviews some of the recent studies on mechanical properties of biological nanocomposites, including their stiffness, strength, toughness, interface properties, and elastic stability. The discussion is focused on the mechanical principles of biological nanocomposites, including the generic nanostructure of hard-mineral crystals embedded in a soft protein matrix, the flaw-tolerant design of the hard phase, the role of the soft matrix, the hybrid interface between protein and mineral, and the structural hierarchy. The review concludes with some discussion of and outlook on the development of biomimicking synthetic materials guided by the principles found in biological nanocomposites.