A crash box of a bumper for a vehicle is provided in the bumper to absorb collision energy produced in a collision. The crash box is constructed such that upper and lower horizontal partitions and a plurality of vertical partitions are connected to each other to form a lattice structure having a plurality of hollow portions, and each of the vertical partitions is formed to be bent at a middle portion thereof. A vertical partition located at the outermost position of the hollow portions forming the lattice structure is bent towards the inside of the crash box, and a vertical partition provided in a middle portion of the crash box is bent to the outside or inside of the vehicle.