The behavior of magnesium (Mg) in the ultrasonic welding process of aluminum (AI) alloy containing Mg was investigated to understand the reason for the poor weldability. Commercially pure Al (A1050) and two classes of Al-Mg alloys which are different in Mg concentration (A.5052 and A5086), 1.2 mm thick each, were lap-welded and the fracture surfaces were examined by SEM observation and AES analysis. For all of these materials, the tensile load of the weld increased with an increase in the clumping load or in the welding time. In the case of Al-Mg alloys, higher Mg concentration required higher clumping load or longer welding time to obtain the tensile load equivalent to A1050. SEM observation of the fracture surface of the weld with A1050 indicated that dimple pattern occupied the almost whole fracture surface. On the other hand, upon Al-Mg alloys, scraped fracture pattern, dimple pattern and brittle fracture pattern were observed and the scraped pattern occupied the largest area in the fracture surface. AES analysis of each fracture surface showed that Mg was segregated at the surface presenting the scraped and the brittle pattern. Especially at the scraped pattern surface, welding for a short time of 0.5 s induced the segregation of Mg about twice as high as the concentration before welding. When the specimen having the surface segregation of Mg formed by the pre-heat treatment was welded, the scraped pattern was observed on the almost whole fracture surface and the tensile load of the weld decreased. Conclusively, it was found out that the surface segregation of Mg induced during the process of the welding caused the ultrasonic weldability of Al-Mg alloy to deteriorate.