Single-discharge test was conducted to characterize the microstructure of electrodes of a high voltage gas spark gap（HVGSG） switch in different discharge conditions. It was observed that the erosion boundary of the electrodes and the dimension of the craters increased with the increase of peak current and transferred charge; the melted state and the small salience due to sputtering of the opposite electrode became evident on the surface of electrodes. The microstructures of Mo electrode and W electrode after several discharges were investigated

and the results showed that: 1)saliences and cracks appeared on the surfaces of both Mo and W electrodes; 2)the Mo electrode exhibits heavier erosion

bigger saliences（30 μm in diameter and 34 μm in height）

bigger and more cracks（20 μm in width and 18 μm in depth）

and more obvious melted state than the W electrode; and 3)the W electrode exhibits lighter erosion

smaller saliences（20 μm in diameter and 18 μm in height）

and narrower cracks（5 μm in width and 8 μm in depth）.