A fixing structure for computer mainboard is provided. The mainboard is clamped between a guard board and a back board. Through the appropriate arrangement of a snap pillar of the guard board and a gourd-shaped hole of the back board, after the snap pillar penetrates the mainboard and the gourd-shaped hole, the mainboard, the guard board and the back board are snapped by moving the back board properly. Furthermore, in the snap state, a fixing pillar penetrates the mainboard and the back board, so as to fasten the two boards. Thus, the step of locking by screws in the conventional art is simplified, and the assembly or disassembly can be fulfilled without any assembling/disassembling tools.