Abstract: In the present paper,the mechanical properties and microstructure of a tortoise shell are studied.The flexural strength in the specimen varies with different parts and loading directions.The maximum flexural strength is 165.1MPa,and fracture toughness is 36.4MPa√m.The shell is comprised of base plates in a rectangle shape.The base plate includes a dense part and loose part.In the dense part,Harversian system is the basic structural unit.There exists Volkmann vessel at its center.The elongated Ca₅(PO₄)₃(OH) crystals are orientationally arranged around the Volkmann vessel.In the loose part,several elongated crystals assemble together and disperse at random,of which the aspect ratio is greater than that in the dense part.There is an organic film between the crystals.The excellent toughness of the shell is attributed to the organic film between crystals and to the organic fibers in the volkmann vessel.