Abstract: The distribution and appearance characteristics of porosities in laser penetrated weld of aluminum alloy were observed, the formation mechanisms of porosities were analyzed in detail, and the influences of twin-spot laser energy distribution on porosities were investigated. It showed that there are two kinds of porosities, metallurgical and technologic porosities, in laser penetrated weld of aluminum alloy. The formation of metallurgical porosities is related to the separation, congregation and incorporation of hydrogen in the weld pool, while instantaneous instability of its keyhole is an essential reason for the occurrence of technologic porosities. Twin spot laser energy distribution can enlarge diameters of the opening and the root of its keyhole, improve fluctuating conditions of the wall of its keyhole, increase stability, and consequently decrease technologic porosities in number, but it has no obvious influence on metallurgical porosities.