Abstract: The material mechanical properties of Kevlar^®29 yarns and filaments with different gage lengths (yarns: 25 mm, 50 mm, 100 mm, 150 mm, 200 mm and 300 mm; filaments: 12.5 mm, 25 mm and 40 mm) were investigated by quasi-static testing utilizing a MTS load frame and a MTI miniature tester. The results manifested that the material mechanical properties of Kevlar^®29 are dependent on the structural scale. The tensile strength decreases with the structural scale altering from fiber to yarn. For filament, the gage length exerts little influence on the tensile properties; but for yarn, the tensile strength decreases with increasing gage length, indicating an obvious size effect. Weibull statistics were used to quantify the degree of variability in strengths of filaments and yarns at different gage lengths. These data were then used in a finite element model considering the probabilistic failure of filaments, which was established by utilizing a user-defined subroutine (USERMAT) in ANSYS to simulate the stress-strain responses of Kevlar 29 single yarn under pseudo-static loading. The predictions of the finite element model agree reasonably well with the experimental data. Parameteric study was also carried out to investigate the effect on the simulation results.