An innovative explanation of spun yarn failure mechanism has been proposed which has validity in explaining some of the anomalies encountered in the previous literatures. A novel mathematical model of yarn tenacity has been derived which can predict the strength of spun yarns produced by any spinning technology. The proposed model of yarn tenacity generates a reasonable agreement between the predicted and measured values of yarn tenacity and it provides a better understanding of the mechanism of failure process of spun yarns. Using the Weibull distribution parameters of yarn tenacity at a particular gauge length and the classical weakest-link scaling theorem the yarn tenacities at other gauge lengths were predicted. The mechanics of failure of various spun yarns were critically analyzed from the investigations of yarn structure, length of failure zone and percentage broken and slipped fibres. The failure behaviors of spun yarns at different gauge length and extension rate were investigated in details. The effect of strain rates and gauge lengths on the characteristics of the stress-strain curves of different spun yarns was also studied.