A packing box includes four walls, a bottom support, a cover and plates. The walls, the bottom support and the cover cooperatively define a space for accommodating items to be packed therein. The bottom support is formed with the walls for supporting the items packed in the space. The cover is configured for closing the packing box. The plates are disposed in the space and are formed by folding a cardboard extension from one or more of the walls. The plates separate the space into compartments.