The sensor to measure the arc length is one of the most basic sensors in order to automate the GMA welding process. In this work, an analytic model of arc light intensity is proposed and the possibility of utilizing arc light as a sensing source is discussed. The relationship between the signal quality and the weaving speed is also discussed. To obtain the welding current and the arc light intensity signal, experiment is conducted on V-groove during the weaving motion. Welding current is set to 240A and 270A, and weaving frequency is set to 0.5Hz, 1Hz, 1.5Hz and 2Hz. In these results, arc light intensity is more sensitive to the change of arc length than that of welding current. When weaving speed increasing, signals having higher quality can be obtained on which the geometry of the work-piece is well-reflected. Also, the model proposed in this study that the arc light intensity have almost linear characteristic to the arc length coincided with the experimental results.