The fine-grained and coarse-grained CuW70 alloys were prepared by sintering and infiltrating. The motion of cathode spots was observed by a digital high speed video camera and a scanning electron microscope (SEM). The results show that the fine-grained CuW70 alloy has larger breakdown strength, longer average arc life and lower chopping current. For coarse-grained CuW70 alloy, the cathode spots ignite and extinguish alternately beneath the anode, the breakdown is mainly located on copper phase, and the erosion pits are much deeper. However, the cathode spots of fine-grained CuW70 alloys have the characteristics of division and deviation, with the uniform distribution of the erosion pits on the entire cathode surface.