A carbon nanotube was bent by Ar ion bombardment. During the process, we used an ion mass doping system for the ion bombardment. As the thickness of carbon nanotube (CNT) changed, the extent of bending varied. The CNT's bending was not caused by the electric field that was applied during the process, but by the defects that were formed on the CNT's wall. We speculate that the defects on the wall changed the hexagonal network into another polygonal network resulting in reduction of the bonding length between carbon atoms. In this paper, we propose a novel method to change the shape of the CNTs and analyze the causes of CNT's bending theoretically and compare our results with those of other groups.