The expansion and shape change behaviors of a single rising bubble and continuously rising bubbles in a reduced pressure vessel were observed. The main results obtained are as follows: 1) The aspect ratio, w/h, of a single rising bubble under reduced pressure is almost the same as that under atmospheric pressure. However, the width of the rising bubble expanded to 1.4 times, which is larger than the calculated expansion ratio. This may be explained by the change of the bubble shape while rising. 2) The observed single bubble shapes were ellipsoidal, spherical cap, and breakup of spherical cap. The upward velocity of the bubble increases with the initial volume of the bubble. The upward velocity also increases as the bubble rises.