A cover structure (1) for a portable electronic device includes a front cover (10) and a back cover (20), wherein the front cover forms a main joint portion (120), the back cover forms a shell (22), the main joint portion has an arcuate wall (121), and a recessed portion (1210) is formed on the arcuate wall. When assembling the front and back covers, a knuckle (12) is formed by means of the shell engaging with the main joint portion on the recessed portion thereof.