The breaking mechanism of nylon-6 yarn is investigated, and the breakage of the yarn under simple tensile loading conditions is compared to that of a filament caused by the same loading. The simple model yarn consisting of seven filaments was prepared for this purpose. The yarn was ruptured at a low rate of extension, and the fracture morphologies of the filaments in the yarn were inspected by a scanning electron microscopy. These morphologies are clearly different from those of the filaments broken individually under the same testing conditions used for the yarn. Various kinds of appearances are obtained in the yarn, for example, a large V-shaped notch is observed, the controlled ductile tearing region is developed across the filament at a large angle to its axis, and the ruptured end is expanded into a plane which is perpendicular to its axis. These causes are discussed in detail to understand the breaking mechanism of the yarn.