Laser welding of aluminum alloys is difficult because of their high reflectivity and heat conductivity. However, high brightness and high power lasers such as fiber laser or disk laser can produce narrow and deep penetration welds, when highly-efficient laser absorption is expected. The objecive of this research is to measure laser absorption in bead-on-plate welding of A5052 aluminum alloy with a 200 μm-fine laser beam spot under a wide range of conditions such as fiber laser powers of 2kW to 10kW or welding speeds of 17mm/s to 250mm/s by water-calorimetric method. Furthermore, the relationship between the laser absorption and the keyhole geometry was investigated by using X-ray transmission in-site observation system or high-speed video camera with diode-laser illumination. It was found that the absorption at 10kW laser power and 17mm/s welding speed was 93% high. Compared with focusing feature of the incident fiber laser beam, X-ray images of the keyhole shape demonstrated that the center part of the incident beam with a bell-shape profile could be delivered directly to the tip of a keyhole. Moreover, the increase in the welding speeds from 17mm/s to 250mm/s reduced the absorption from 93% to 72%. The highspeed observation pictures showed that the incident fiber laser beam was partly shot on the molten pool in front of the keyhole inlet at higher welding speeds, which might be the chief cause of the decrease in the laser absorption. Consequently, the tightly-focused high-power fiber laser welding was confirmed to be a highly-efficiency process of 93% to 72% in laser absorption.