The variance of fiber morphology along a fiber and the natural and artificial flaws in the fiber structure represent the primary reasons for the weak link of fibers. Accordingly, the fiber weak link can be divided into two types, that is, the geometrical thinnest part and the structural weak point. Scanning electron microscopic observation was used to characterize the morphological features of the fiber weak points whose forms are the normal thin sections, natural flaws, and artificial damage. Both the fiber profile morphology and the tensile behavior of wool fibers have been measured using a single-fiber analyzer (SIFAN) and an optical microscope with a CCD camera plus an XQ-1 fiber tensile tester (OM + XQ). The results from the SIFAN and CM+XQ methods indicate that the fibers breaking at their minimum diameters represent only one part of the broken fibers. The percentage of this kind of breakage is in the range of 40-60%. A new approach is presented to identify the weak-point breakage relying on the fiber tensile behavior. The experimental results show that the probabilities of weak-point, normal, and thinnest-part breakage evaluated by these methods approximate 40, 60, and slightly more than 80%, respectively. (C) 2003 Wiley Periodicals, Inc.