A brake disk assembly for a bicycle has a body, two brake pads and an adjusting device. The body has a slot, and the brake pads are operationally mounted inside the body. The adjusting device is mounted on the body and has two wings, two adjusting plates, a connecting arm, two bolts and a concave-convex configuration. The wings extend from the body. The adjusting plates are attached respectively to the wings. The connecting arm is mounted between the adjusting plates and has two threaded holes defined respectively in two ends of the connecting arm. The bolts extend respectively through the wings and the adjusting plates and are screwed respectively into the threaded holes in the connecting arm. The concave-convex configuration is mounted between the wings, the adjusting plates and the ends of the connecting arm to make the body adjustably connectable with the connecting arm.