There are two main types of bright dot defects in TFT LCD where liquid crystal is aligned by rubbing process, one is with particles and the other is without particles. The former can be reduced by enhancing cleaning, but the way to reduce the bright dots without particles is barely studied, so research and experiments are performed. According to macro and micro result, the alignment of liquid crystal is abnormal because of impurities as tall as 7.5~10 nm in the defect area. It can be deduced that the abnormally alignment is caused by the remaining of the back coating and the softener agent on the PI surface. Lastly, the rubbing cloth, in which the back coating and the softener agent are reduced, is tested in our factory. The results show that the bright dots w/o particles decrease from 06% to 0% when the back coating and the softener agent in the rubbing cloth are reduced, which proves the effectiveness of the laboratory results and the correction of the mechanism.