Micron-sized truncated octahedral LiNi0.5Mn1.5O4 (LNMO) samples with different degrees of Ni/Mn disordering have been obtained by controlling the synthesis conditions, such as calcination atmosphere (O-2 and air), cooling rate or additional annealing step. The influences of Ni/Mn disordering on the physical properties and electrochemical performance of the truncated octahedral LNMO samples have been systematically investigated. The analyses of thermogravimetry, X-ray photoelectron spectroscopy, X-ray diffraction, powder neutron diffraction, Raman spectroscopy and X-ray absorption spectroscopy reveal that the occurrence and degree of Ni/Mn disordering are closely related with the formation of oxygen vacancies and presence of Mn3+. Slow cooling rate and post-annealing can result in low degrees of Ni/Mn disordering and oxygen vacancies. Electrochemical measurements show that Ni/Mn disordering and oxygen vacancies have no obvious effect on the rate capability since all LNMO samples share a truncated octahedral morphology with the exposed {100} surfaces. However, they play significant roles in improving long-term cycling stability, especially at the elevated temperature of 60 degrees C. This work suggests that the electrochemical performance of LNMO with optimized truncated morphology can be further enhanced through tuning the degrees of Ni/Mn disordering and oxygen vacancies. (C) 2018 The Electrochemical Society.