Experimental models which are able to determine the deviation between weld line and weaving center by measuring the weld current during welding were proposed for the gas metal arc welding process. The models were used for developing a weld seam tracking system which controls the weaving speed of a welding torch. However, it was revealed that the tracking result of the system is affected by the welding conditions. Thus an arc sensor system was developed by using fuzzy control approach for overcoming the difficulty of modelling the nonlinear process. The rule base and parameters of the fuzzy control system were determined on the basis of the results of experiments. This fuzzy control system has shown the successful tracking capability for the wide operating range of welding conditions.