The fracture of lithium nickel manganese cobalt oxide (NMC) particles can affect the electrochemical performance and manufacturing process of NMC electrodes. Here, we report the fracture behavior of single LiNi0.33Mn0.33Co0.33O2 (NMC111) secondary particles studied by flat punch indentation. The critical load corresponding to the fracture of NMC111 secondary particles increases with increasing particle size, while the fracture strength is unaffected by the size of the secondary particles. Particles at the first delithiation and lithiation states have significantly lower critical load and fracture strength than the pristine ones, suggesting that electrochemical cycling strongly affects the mechanical integrity of NMC111 secondary particles. (C) 2019 The Electrochemical Society.