Low frequency internal friction in the process of plastic deformation of Al-Cu and Al-Mg alloys was measured on a small tensile test machine and a series of internal friction jumps was observed on the internal friction-strain curve. A study was made on the effect of temperature of measurement and some metallurgical factors on these internal friction jumps. Experimental results showed that the range of the temperature of measurement within which these internal friction jumps appeared corresponds tc that for the appearance of the serrated stress-strain curves of aluminium-copper alloys. This indicates that these internal friction jumps on the internal friction-strain curves are associated with the process of strain aging occured in the process of plastic deformation, i.e., they are originated from the alternate anchoring of dislocations by atmospheres of solute atoms and their subsequent escaping in the course of plastic deformation.