In this study, size effect of NiO particles has been studied as the anode material of lithium ion battery. It is found that NiO nanoparticles synthesized in confined space of ordered mesoporous silica behave anomalous high capacity and higher energy efficiency than sub-micro-sized NiO does. The higher energy efficiency is resulted from the reduction of the hysteresis loop between charge and discharge voltage plateaus, the main drawback of conversion reaction-based metal oxide anode materials. The interesting behavior is proposed to be the reversible formation/dissolution of solid electrolyte interphase (SEI) layers associated with 3-D porous, originally nanostructured NiO electrode that is able to remain stable during charge–discharge process.