Abstract: In order to investigate the size effects and strain rate sensitivity of Kevlar 49 single yarn, quasi-static (strain rate is 1/600 s^-1) tensile tests were conducted on Kevlar 49 single yarns with different gauge lengths (25, 50, 100, 150, 200 and 300 mm) using MTS universal testing machine firstly. Then, dynamic (strain rate is 40-60 s^-1) tensile tests were also conducted on samples whose gauge length is 25 mm by Instron drop-weight impact system. Finally, Weibull model was used in statistics analysis, and the degree of random variability in tensile strength of Kevlar 49 single yarns under different gauge lengths and strain rates was quantified. The results show that the tensile mechanical properties of Kevlar 49 single yarns have correlation with gauge length and strain rate. Tensile strength decreases with increasing gauge length, but increases with increasing strain rate. While ultimate strain and toughness both decrease with increasing gauge length and strain rate. The extract Weibull parameters can be used in numerical simulation and engineering application.