Variations of internal friction in the course of strain-aging at room temperature of slightly cold-worked Al-0.1% Mg alloy were measured with a torsion pendulum and an internal friction peak (versus aging time) exhibiting anomalous amplitute effect was observed. When the internal friction of a fully aged specimen was measured with the temperature going up and down, internal friction peaks (versus temperature) exhibiting anomalous amplitude effect were also observed in the temperature range of 30-55℃. These anomalous effects were not observed in specimens of high purity aluminum or fully-annealed Al-0.1% Mg alloy.The fact that these anomalous internal friction phenomena previously observed in Al-0.5% Cu alloy can also be observed in Al-0.1% Mg alloy excludes any possibility of explaining these anomalous internal friction phenomena in terms of a precipitation process which cannot occur in Al-0.1% Mg alloy in the temperature range of the present measurements.In order to explain the characteristics of the anomalous internal friction phenomena observed in the aluminum-magnesium alloy, two different types of dislocation atmospheres were assumed to be able to exist in an aluminum-magnesium alloy: One type is concerned with the segregation of "magnesium atom-vacancy" pairs to a dislocation, which can happen during the course of strain-aging; another type is concerned with the segregation of magnesium atoms to a dislocation, and these results when the cold-worked specimen was heated to a temperature around 100℃, at which temperature magnesium atoms were freed from the vacancies and so are able to form an atmosphere alone with a dislocation when the temperature is lowered. The pinning of a dislocation by an atmosphere of the former type is stronger than that of the latter, so that the amplitude internal friction peak associated with the former appeared at a higher amplitude level.