In this study, a 6 mm-thick 6061 T6 aluminum alloy was welded using fiber laser welding, and its mechanical and metallurgical properties were investigated. Full penetration autogenous welds were made at a constant laser power of 4.95 kW using a continuous-wave fiber laser and focal positions of -4 mm. In addition, the shielding gas content was changed for weldments welded at different travel speeds (mm s(-1)). Accordingly, welding speeds were 50, 40, 35, 25, and 20 mm s(-1), and the shielding gas contents were 100% argon (Ar), 10% helium (He) + 90% Ar, 30% He + 70% Ar, 50% He + 50% Ar, and 75% He + 25% Ar. Then, macro and micro images were taken, and some mechanical properties (hardness, tensile test, and bending test) were examined. As a result, the change in shielding gas content and welding speed has provided significant variations in the welding joints. Decreases in mechanical properties were detected due to the coarsening of the precipitates. On the other hand, the narrowness of the weld seam and HAZ obtained in laser welding, compared to traditional welding methods, minimizes this disadvantage.