A casing of an electronic device includes a shell composed of a number of unit bodies arranged in a number of unit body layers. Each of the unit bodies is a hollow structure including a main portion and two end portions. The two end portions are located at opposite ends of the main portion. The main portion is a hollow hexagonal column. Each of the end portions includes three rhomboid faces extending from the main portion and forming a tip. The unit body layers are arranged such that the tips are staggered.