In a welded structure, thick plates are joined by multi-pass welding in the butt joint. During the first pass of multi-pass welding, burn-through, lack of fusion, and incomplete penetration were readily found as weld defects. Accordingly, the backing condition should be carefully selected in welding of a thick plate, because improper backing conditions lead to weld defects. In the job site, a steel backing strip is usually adopted, although it reduces the fatigue strength. No backing conditions or removable backing is recommended to increase the fatigue strength, but selection of the proper backing and welding conditions is complicated. In this study, several backing methods, such as ceramic backing, water-cooled copper backing, and even the use of no backing, were investigated during GMA (Gas Metal Arc) welding of a thick steel plate. The gas metal arc welding conditions were established for each backing method, and the bead shape and mechanical properties were examined after welding.