Abstract: In order to investigate the tensile mechanical properties of Kevlar 49 single filament (microcosmic) and single yarn (mesoscopic), MTI miniature tensile tester and MTS microcomputer control electron universal testing machine were used respectively to conduct the unidirectional tensile tests on filament and yarn firstly, and found that there was a significant correlation between the mechanical properties of filament and yarn as well as gauge length and structural length scale of samples. The tensile strength of samples decreased with the increasing gauge length and structural length scale enlarged from filament to yarn. Then, the testing data was statistically analyzed according to Weibull distribution, and the random variability degree for tensile strength of filaments and yarns under different gauge lengths was quantified. After that, for the tensile strength of filament according with the Weibull distribution and probabilistic failure, user-defined subroutine (USERMAT) in ANSYS was used to establish the filament constitutive model. Finally, the filament constitutive model was used to simulate the tensile failure behaviors of yarn, and the effects of key parameters on tensile failure behaviors of yarns were also discussed. The results show that the goodness of fit between simulated results and testing results of yarn is preferable, and the established USERMAT can predict the tensile properties of yarn comparatively accurately.