Abstract: In this research, we have studied the fatigue behavior of sized warp yarns under cyclical elongation as measured on a Lloyd dynamometer on the basis of damage rate. Fatigue behavior expressed in terms of cycles at failure and the rate of fatigue damage expressed in terms of loss in tensile properties. We have found that deterioration of tensile yarn properties increase with increasing fatigue level. Comparing fatigue behavior of three different sizing agents, PVA have the less damage in his tensile properties. A theoretical model is developed to predict changes in load under longitudinal loading subjected to extension cycling. The model gives the best agreement with experimentally tensile cycling of sized yarn cycled between two base tension limits.