We reported the effects of grain size on high dielectric and related electrical properties of Li0.05Ti0.02Ni0.93O (LTNO) ceramics, which were prepared by a direct thermal decomposition method. The analysis of complex impedance indicated that these LTNO ceramics were electrically heterogeneous consisting of conducting grains and insulating grain boundaries (GBs). Interestingly, our results revealed that the dielectric permittivity (ε′) increases with the increase in grain size, which can be well described by Maxwell–Wagner relaxation model. Furthermore, we also found that the activation energy required for relaxation process (Ea) and related activation energy of the conductivity in the grain interior (Eg) decreased with the increase in grain size. These results suggested that the different microstructures resulted in chemical change (e.g., oxygen vacancies) inside the grains, leading to the changes in electrical properties of the LTNO ceramics.