The microstructure evolution during microwave-assisted sintering and spark plasma sintering of 50 μm spherical powder is presented. The two microstructures formed are alike, and show unexpected details that cannot be explained just by conventional thermal and pressure effects; one also has to consider “field” effects. This also suggests that the driving force is similar. An explanation and analysis of the microstructure evolution is attempted based on the enhancement of the conductivity of zirconia when the heating temperature increases. The consequences are possible discharge effects at lower heating temperatures and high reversed heating gradients above 800–850 °C.