This investigation examines the grain boundary segregation of Mg and Si in AA6061-T4 alloy, using Auger electron spectroscopy technology. Liquid metal embrittlement by gallium was conducted to fracture the AA6061-T4 alloy intergranularly to obtain compositional information directly from the grain boundary facets. The amount of liquid gallium applied is controlled to break the alloy intergranularly at room temperature, importantly without Ga-bearing particles or a film covering the fracture surface. The method of generating a fully intergranular fracture surface for AA6061 is elucidated. The AES analysis reveals that Mg in 6061 T4 alloy is segregated at grain boundaries, but Si does not. The segregation of Mg depends on the rolling direction. The mean peak-to-peak ratio I_Mg/I_Al of the specimen whose longitudinal axis is perpendicular to the rolling direction is about three times that of the specimen whose axis parallel thereto. The grain boundary segregation is not result oxidation-induced; surface segregation also makes no contribution.