Aerodynamic damping on vibration of rotating disks surrounded by a shroud was experimentally investigated. Hammer tests were conducted in a vacuum and in air, and a curve-fit technique was used to estimate natural frequencies and modal damping ratios of disk modes precisely from measured frequency responses. The experimental results revealed that aerodynamic damping is dominant rather than structural damping and independent of the amplitude of the disk vibration. The relationships of aerodynamic damping with disk-to-shroud and disk-to-cover gaps were also investigated. When the disk-to-shroud gap is narrower than 0.3 mm, the damping ratios increase rapidly as the disk-to-shroud gap narrows and the rate of increase depends on the disk mode. For the (0, 0) mode, the damping ratio is inversely proportional to the cube of the disk-to-shroud gap, which agrees with our previous theoretical prediction. In contrast, when the disk-to-shroud gap is wider than 1 mm, the rate of increase is slow and varies insignificantly with disk mode.