We report the failure mechanism of a Ni-rich layered cathode material (LiNi0.8Co0.1Mn0.1O2) in Li-ion full cells. Laminated pouch-type full cells were cycled 300 times, after which the cells exhibited 16.3% capacity fading. After 1, 100, 200, and 300cycles, each full cell was disassembled and the cathode was investigated via a half-cell test, X-ray diffraction (XRD), and field emission scanning electron microscopy (FE-SEM). XRD analysis showed loss of the active lithium source in the cathode, and FE-SEM observation exhibited traces of mechanical failure in the Ni-rich secondary particles. Particle strength measurements on the cathode materials by applying compressive force demonstrated that the mechanical strength of particles significantly weakened after full-cell cycling. Thus, the capacity of the Ni-rich cathode may deteriorate during full-cell cycling because of decreases in compressive particle strength. This issue should be considered in materials preparation processes.