The diffusing behaviors of small hydrocarbon radicals (-CH, 2CH 2 and 2CH 3 ) on the outer wall of a รฐ9; 0ร carbon nanotube have been investigated by performing the tight-binding potential calculation. We find that either -CH or 2CH 2 preferentially zigzags on the wall of the tube, whereas 2CH 3 hops from the top of a carbon atom to its nearest one, by overcoming energy barriers of less than 1.3 eV. If 5-1DB defect, a type of monovacancies, exists in the tube, it seems a trap to capture the radicals diffusing nearby. Two typical pathways of the radicals migrating into a 5-1DB defect are revealed in this work. In one of the pathways, -CH and 2CH 2 diffuse from the defect-free region to the dangling bond (DB) atom in the 5-1DB defect by crossing the hexagon directly. In the other one, the radicals diffuse to the pentagon, where either -CH or 2CH 2 converts the initial 5-1DB defect to be an overturned one (by breaking and rebonding C-C bonds), with -CH or 2CH 2 attached on the new DB atom. Furthermore, we find that -CH or 2CH 2 bonding at the DB atom can ultimately convert the 5-1DB defect to be a three-fold coordinated or bridge-like structure only with energy cost of 0.5 eV, improving the local structure of the monovacancy. However, a 2CH 3 radical cannot improve the structure of the 5-1DB defect anyway.