A hub assembly for a disk brake of a bicycle includes a support shaft, a hub body pivotally mounted on the support shaft and provided with two drum disks, a mounting portion protruding from one of the drum disks, a threaded section protruding from the mounting portion, a brake disk resting on the end face of the mounting portion, and a chainwheel screwed onto the threaded section and pressing the brake disk so that the brake disk is clamped between the mounting portion and the chainwheel closely. Thus, the brake disk is mounted on and detached from the hub body by screwing and unscrewing the chainwheel, so that the hub assembly is assembled and disassembled easily and rapidly, thereby facilitating a user assembling and disassembling the hub assembly.