Abstract The investigation of GdBaCo2O5+δ system using low frequency internal friction method indicates that there exists a relaxation internal friction peak originated from the jump of excess oxygen atoms. The excess oxygen content δ affects greatly the height, shape and position of the peaks, which reflects the state of the excess oxygen varying with the value of δ. When the excess oxygen content is very small, say for the sample with δ=0.005, the corresponding internal friction peak disappears. On increasing the excess oxygen the relaxation internal friction peak will be observed. Three different states of excess oxygen can be concluded from the relaxation peaks which exist in the samples with δ=0.278,0.407,0.421,0.431 and 0.515. In addition, an internal friction peak with the features of phase transition are also observed in samples with δ=0.421 and 0.515 at about 360K, which may be attributed to metal-insulator transition.