A 100 kW fiber laser was first used to weld steel. Speeds at the range between 0.3 and 5.0 m/min were tested, and the maximum weld bead depth of 70 mm was achieved by single pass welding. Solidification cracking and porosity occurred when the welding speed was lower than 0.5 m/min, while undercut appeared when the welding speed was higher than 3.0 m/min. Both the ratio of depth to width and the cross section area of the weld bead had a positively linear relationship with the welding speed. A high speed camera was used to observe the characteristics of the keyhole and molten pool. The average number of spatters increased with the welding speed, while the keyhole diameter and the length of the molten pool in front of the keyhole decreased with the welding speed. This Letter validates the application potential of a 100 kW ultra high power fiber laser in manufacturing, e.g., welding, cutting, and additive manufacturing.