Electrical breakdown process of ceramics is a complex process related to electrical, thermal, and light effects. It has attracted lots of attentions. But the mechanism of dielectric breakdown is still under debate, especially for the solid ceramics. In this study, the ceramics with different grain size were sintered at different temperatures. A square pulsed voltage of 27 kV with the pulse width of 7 ms was generated to breakdown the ZnO ceramics with a thickness of 2 mm. Electric breakdown process of these ceramics were investigated using a homemade high-time- resolution electric detection system with the resolution < 1 ns. Compared the electric behaviors of ZnO ceramics with different grain sizes during the breakdown process, it is clear that the whole electric breakdown process could be divided into three main steps, all of which can completed within 7 ms. The first breakdown step is the pores breakdown due to the electric field concentration, and the second step is the grain boundaries breakdown, and then the bulk grain is broken-down leading to form the whole electrical breakdown channel. The data shows that the breakdown process of the grains has the longest duration, the grain boundary is the second, and the pores breakdown is the shortest. For the ceramics with different grain sizes, the breakdown durations of the grain boundaries are different. The duration and electrical breakdown speed of the three step processes are directly affected by the resistance uniformity and carrier concentration of the materials.