Abstract: SEM observation on a clam’s shell showed that the shell is a kind of bioceramic composite consisting of aragonite layers and collagen protein matrix.The aragonite layers consist of long and thin aragonite sheets parallel with the surface of the shell.The observation also showed that the each aragonite sheet consists of lesser aragonite sheet fibers.The thickness of the aragonite sheet fibers is of nanometer scale.The aragonite sheet fibers in different aragonite layers possess different orientations,which compose a kind of intersectant nanostructure.The maximum pullout force of the aragonite sheet fibers in the intersectant nanostructure was investigated and compared with that of the aragonite sheet fibers in nanostructure.It showed that the intersectant nanostructure can markedly increase the maximum pullout force of the aragonite sheet fibers and improve the fracture toughness of the shell.