A tubular doorknob includes an inside bracket, an outside bracket and adjustment devices. The outside bracket has two connecting wings, two outside passages, two outside screw holes and an outside adjustment device. The connecting wings protrude from diametrically opposite sides of the outside bracket. The outside passages are defined through the connecting wings. The outside screw holes are defined through the outside bracket and communicate with the outside passages. The outside adjustment device has two sliding blocks and two adjustment screws. The sliding blocks are slidably mounted in the outside passages. The adjustment screws are screwed into the outside screw holes to push the sliding blocks to abut a surface in a doorknob hole in the door. The inside bracket has a configuration similar the outside adjustment device. The extended adjustment devices firmly hold the tubular doorknob in the doorknob hole.