A fault of metal oxide varistor(MOV) for series compensation capacitor banks is analyzed in this paper. The MOV is checked by visual inspection and electrical test, and the wave of fault recorder is analyzed incorporating the simulation. The fault is due to the discharge the capacitor, because the MOV is broke down while the phase - phase grounding fault appear. According to this fault, two improvement measure(change MOV not at will, replace MOV with a long history and performance degradation) are presented, so as to improve and enhance operation and maintenance situation of the MOV.