Arc sensor is a conventional technique toward the compensation of aiming deviation which causes from the heat distortion or unevenness of work piece. But, at present, only less than 5% of arc welding robot has arc sensing function. It is hard to use in actual welding line because of occurring of miss sensing. Many studies have been performed about arc sensing, but most of them are about the performance of sensing. So, we began to reconsider over arc sensing in the meaning of reliability. We will apply theoretical equations so as to separate the signal and the noise toward unsettled arc phenomena. This report shows the research toward the behavior of arc itself at first. We developed an averaging method of arc voltage by using histogram so as to get accurate arc length. Next, we research the effect of molten pool toward the sensitivity of arc sensor. Wider weaving width leads better sensitivity. But even if the weaving width becomes small, we found the noise level does not increase.