Laser-micro-welding based on galvanometer scanning has attracted great interest. Herein, bead-on-plate scanning micro-welding of 100-μm-thick AISI304 stainless steel foil is conducted by using a single-mode fiber laser equipped with a galvanometer scanning system. A comparative study is conducted on the laser-micro-welding processes with/without Ar-gas protection, and process windows of laser-micro-welding for the both conditions are established. Results show that an unstable transition zone exists during laser-micro-welding without gas shielding. The unstable transition zone is generated during the deep penetration welding mode alone, rather than during the alternation of thermal conductivity welding mode and deep penetration welding mode, as reported previously. The oxidation during laser-micro-welding results in an unstable deep penetration welding mode, during which a transition zone is formed. Furthermore, by applying gas shielding, we find that the unstable transition zone is eliminated, and the welding-process window is expanded.