This paper has investigated flashover performance of alumina insulators prepared with different additives in vacuum under an impulse voltage. Four kinds of alumina insulator were studied, which were prepared under the same sintering conditions with different additives. The surface flashover performance of these samples has been tested in vacuum. The microstructure of alumina samples has also been observed using the technique of scanning electron microscope (SEM). Based on the experimental results, the characteristics of flashover are different from the test samples. The analysis shows that the influence of additives may be attributed to the change of microstructure.