The purpose of this study is to gain an understanding of parameters that affect the strength of yarns. Quasistatic and dynamic strength of five different yarns were obtained using hydraulic and Hopkinson bar testing methods and the rate dependency of the failure strength of each yarn was quantified (average of 10 repeats). The scaling effect was also studied experimentally in order to relate the effect of specimen size to the failure stress of Kevlar 129. Single fibres were tested at five different gage lengths (5, 16, 25, 50, 100 mm) and multi fibre specimens were also tested at various gage lengths (24, 100, 170 mm). By studying the variation of statistical strength with increasing size of specimen, a decreasing trend for the tensile strength was observed. A three-parameter exponential growth model is proposed to relate the strength of the specimen to its volume in order to incorporate the increase in defect population.