A heat dissipating structure for a computer casing includes a computer casing having a front board, a back board, and a bottom board, wherein the front board and the back board are parallel and correspond to each other by being connected together on the bottom board. There are two supporting frames mounted at an opposite side corresponding to the bottom board for respectively connecting with the upper sides of the front board and the back board so as to form a rectangular hexahedron. There are plural placing slots mounted on the front board for placing a disk drive, a CD-ROM drive, and plural connectors therein, wherein the front board further has the plural heat dissipating openings mounted thereon between the positions of the placing slots. Through the heat dissipating openings, the air flowing outside is drawn into the inside computer casing so that the heat generated by the electrical elements can be exhausted to the outside and a better heat dissipating effectiveness is achieved.