Fully dense, doped ZnO varistors were prepared using an easy two-stage pressureless-sintering method at temperatures as low as 825degreesC, with a grain size of similar to0.5 mum. After the highly nonohmic ZnO varistors were sintered, their fine microstructure consisted of uniformly sized grains, small spinel grains with partially dissolved manganese and cobalt oxides discontinuously distributed in the fairly wide grain boundaries, and an intergranular layer of bismuth-rich crystalline phase mainly detected at three or four ZnO grain junctions. There were twins near the middle of almost all the ZnO grains. The abnormally high nonlinear properties of the almost nanostructured varistors (F-B approximate to 6-8 kV/mm and alpha = 270) were attributed to a uniform and very fine microstructure, a high ZnO-ZnO grain direct contacts concentration, and a uniform hybrid layer substructure (grain boundaries and twin boundaries) with different (but probably accumulative) potential barriers.