Abstract: The authors developed an arc direct visual sensor and image processing algerithm by means of which the on-line detection of the narrow gap weaving MIG/MAG process can be performed successfully. As the speed of image pickup and processing come up to the level of 20~30 images per second, the locus of wire tip transverse weaving can be detected, and the width of weaving as well as the asymmetric value of wire weaving with respect to the sidewalk can be calculated. These results may be used as the ideal signals for controlling the weaving of arc automatically. This technique is also a new useful means of investigating the influence of arc weaving on the welding parameters, shape of molten pool and weld quality, together with their realtime control.