A low‐frequency internal friction peak is observed in cold‐worked Al‐0.5 wt% (0.21 at%) Cu around 214 K (f = 2 Hz). Effect of annealing temperature, measurement frequency, strain amplitude, and cold‐working condition on the peak temperature, the peak height, and the peak width of this peak is systematically studied. This peak behaves anomalously in comparison with the low‐temperature peak previously observed in cold‐worked dilute AlMg and AlGa alloys in that the peak temperature increases first with an increase of measuring frequency when the annealing temperature is below 400 K, but becomes constant when annealed above 400 K, and that this peak is very broad. These peculiarities led to the assumption that this peak consists of two closely situated internal friction peaks: the lower‐temperature component is a relaxation peak around 206 K and the highertemperature component is a frequency‐independent “phase transformation peak” around 224 K which may be attributed to some clustering state of copper atoms around dislocations just before the formation of GP zone. The detailed analysis of this peak will be reported in a subsequent paper.