Relaxation of a yarn is defined as the change in stress in a yarn with regard to the time during which strain is kept constant. Relaxation of yarns should not be neglected as it can cause problems during processing of yarns. It is known that relaxation of warp yarns during loom stops is one of the factors influencing the occurrence of set marks. A test method was presented for measurement of the relaxation of yarns after cyclic loading. Three types of relaxation curves are possible. Test parameters (number of load cycles, frequency of the dynamic loading, stress level at which relaxation starts) and fibre raw material have a high influence on the relaxation behaviour of spun yarns. The strain, at which the relaxation test is executed, has a large influence on the residual force, in accordance with the residual force after infinite time calculated using a model for the description of the stress-strain curve of spun yarns.