For industrial polyester yarns, the study of hydrolytic/thermal degradation is important, because Of the negative influence of these degradations on mechanical properties of polyester yarns. In this research, tensile force at breaking point and elongation at break were used to assess the hydrolytic and thermal degradation of industrial polyester yarns with two draw ratios of 3.5 and 4, which were exposed in water and in oven for 28 days and at different temperatures of T-g, below and above T-g, Totally, the amount of these degradations at temperatures of 70 to 90 degrees C is not high. The investigation shows that tensile strength at breaking point of industrial polyester yarn will decrease in water and in oven at higher temperature; this degradation is lower for yarn with high draw ratio. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 1929-1932, 2010