The authors have observed directly and simultaneously the laser-induced plasma and keyhole behavior by high speed frame/streak camera using a special optical systems during the pulsed YAG laser welding of Al-Mg alloys in air and argon atmospheres. The dynamic behavior of Al-Mg alloys plasma was very unstable and this instability fluctuation period was about 440µs. After laser termination, abrupt collapse of keyhole within 600µs was observed and it was related to the formation of porosity. The authors also performed the spectroscopic analysis of laser-induced plasma, and clarified the structure and composition of evaporated particles during pulsed laser welding of Al-Mg alloys. In the air environment, the intensities of molecular spectrum of AlO and MgO were different each other depending on the power density of laser beam. Under the low power density irradiation condition, the MgO band spectrum was predominant in intensity, while the AlO spectrum became much stronger in higher power density. The same result was revealed in analyzing the composition of evaporated particles. These results were attributed to evaporation phenomena of metals with different boiling points and latent heats of vaporization.