Abstract: The stress relaxation properties of yarns are studied.Based on the superposition principle of Boltzman,the elongation-stress relaxation process of yarns is analyzed using the standard linear solid model.Theoretical equations of the elongation-stress relaxation process are deduced.The elongation-stress relaxation experiments of polyester/rayon and regenerated bamboo fiber/cotton blended yarns are performed under such conditions as constant stress and different elongation rates and constant elongation rate and different stresses.The regressive equations of stress relaxation and correlation coefficients of the yarns are calculated.Theoretical expectations display very good agreement with the experiment observations.It indicates that the standard linear solid model can be used to describe the stress relaxation properties of yarns under lower strain conditions.It is found from the experimental and theoretical results that the higher the strain or the elongation rate,the lower the relaxation time and more apparent the relaxation phenomenon.