Crab carapace is a kind of biomaterial in nature. It behaves favorable strength, stiffness and fracture toughness, which are closely related to its fine microstructure. The observation of scanning electron microscope (SEM) on the carapace of a Cyclodorippoidea crab shows that the carapace is a kind of natural bioceramic composite consisting of calcite crystal layers and collagen protein matrix. The observation also shows that the calcite crystal layers consist further of long and thin calcite crystal sheets and that all the calcite crystal sheets are arranged in a kind of parallel distribution. The maximum pullout energy of the calcite crystal sheets, which is closely related to the fracture toughness of the carapace, is investigated based on the representative model of the parallel distribution. It shows that the long and thin shape as well as the parallel distribution of the calcite crystal sheets enhance the maximum pullout energy and ensure the high fracture toughness of the carapace.