We propose a model for internal friction at low frequency in fec alloys due to extended dislocations is proposed, in order to explain the internal friction peak around 210 K in heavily cold-worked concentrate Cu-Al and Cu-Zn alloys with measuring frequency of about 1 Hz.The motion of extended dislocations under the action of applied stress is divided into the relative motion (the relative displacement between two partial dislocations) and the overall motion (the displacement of the centre of extended dislocations). In addition to the restoring force due to the interaction between two partials and the restoring force due to the line tension of dislocations, the restoring force due to long range interaction among neighbouring dislocations is introduced. It is demonstrated that the last restoring force only influences the overall motion of extended dislocations, but does not influence their relative motion.The relative motion and the overall motion of extended dislocations each give rise to its internal friction peak respectively. The two peak temperatures are close to each other. The observed internal friction peak is a superposition of these two peaks. The predictions of the model in the present paper are in agreement with the experimental results.