Recently, the welding of aluminum alloy has been strongly required for lightening the electronic products, automobiles and so on. Then, hybrid laser system was newly developed in order to perform the high efficiency and high quality welding of aluminum alloy. We have developed the hybrid system which is consisted of a continuous diode laser and a pulsed Nd:YAG laser. In this system, high electric-optical conversion efficiency was achieved by the newly developed diode laser pumping system, and it can deliver two laser beams by using only one fiber. Higher absorption of laser energy to the aluminum alloy can be expected under the continuous irradiation of diode laser (wavelength: 808nm), since the absorption of light to aluminum alloy shows the maximum at 850nm. When the pulse waveform took the highest peak at the first period, the suitable bead width and depth could be obtained with better surface integrity. The bead width and depth increased with increasing the power of continuous diode laser, which led to overlapping welding even under the high speed condition. Numerical calculation made it clear that the surface temperature of aluminum alloy with diode laser could keep 300K higher than that without diode laser before the next laser shot. Therefore, it could be expected that pulsed Nd:YAG laser would be absorbed more effectively because of higher surface temperature of aluminum alloy, and high efficiency processing would be performed. Moreover, the high efficiency and the high quality welding of aluminum battery case could be accomplished by this hybrid system, in which the suitable pulse energy and the peak power could be given even under the high pulse repetition rate.