The mechanism of the increasing of resistance of lithium-ion batteries using Li(Ni,Co)O2 based material as positive electrode materials was investigated. The increase in resistance of cells with Li(Ni,Co)O2 material and artificial graphite was observed in cycling test at 60°C. It was found that the increasing of resistance was mainly attributed to that at positive electrode by ‘reconstructed method’. In order to examine the mechanism of the increasing of resistance of positive electrode, the morphological changes of positive particles and local structure of Li(Ni,Co)O2 material were investigated. The clearance and crack were observed at grain boundary inside particle after cycle by FIB (focused ion beam) technique. The change of local structure around Co-atom was observed during charge–discharge process at 60°C by in situ XAFS (X-ray absorption fine structure spectroscopic) analysis method. It was considered that the morphological and local structural changes of positive material cause the increasing of resistance.