Abstract: In robot arc welding process, occasional disturbance will lead to uneven welding quality and weld defects. The on-line welding quality monitoring becomes one of the most heeded technology in the production industry especially in the automobile component manufacture. In this paper, a technology for on-line monitoring weld quality in robot arc welding process is described.It is based on the are signal features in the welding position, heat input and defects, to get the weld quality information and to evaluate if it is acceptable. It is important for "zero defect" quality control in robot welding production.