Li+ ionic diffusion in LiCoO2 exposed to ball milling was studied by XRD, complex resistivity, and NMR. Angular frequency dependence of the complex resistivity is analyzed by using a Debye-type relaxation model. The Li+ ion hopping time becomes fast by exposing to short-time milling, whereas it becomes slow by exposing to long-time milling. The activation energy of the Li+ ionic diffusion in milled LiCoO2 samples varies from 0.29 to 0.36 eV as the milling time increases up to 60 h. These results suggest that exposure to appropriate milling time enhances the Li+ ionic diffusion but the excessive milling is disadvantageous for the Li+ ionic diffusion. In addition, an increase in the line width of Li-7 NMR spectrum with increasing milling time is discussed in connection with the line width arising from magnetic cobalt ions. (C) 2008 Elsevier B.V. All rights reserved.