Micropore growth and void initiation and growth behaviors of 5052 aluminum alloys during tensile deformation were observed by means of X-ray computed tomography (CT) using the X-ray image beamline (BL20XU) of the Japanese synchrotron radiation facility (SPring-8). Tensile tests were carried out in ambient and wet conditions. In 5052 alloy tensile tested in wet condition, the drop of stress followed by the maximum stress was remarkable, and the fracture strain was reduced in comparison with those of 5052 alloy tested in ambient condition. The number of fine micropores and voids decreased in 5052 alloy when plastic deformation started and the decrease in the amount of fine micropores and voids in the wet condition was less than those in ambient condition. These might be caused by the acceleration of micropore growth and void initiation and growth during tensile testing in wet condition.