We have been studying the breakdown characteristics of polyimide (PI) thin films to try and take advantage of self-healing breakdown. This paper focuses on the effect of ambient gas (SF/sub 6/, N/sub 2/, O/sub 2/ and dry air) on breakdown characteristics of PI thin films. At room temperature, electric strength (Fb) in SF/sub 6/ is higher than in other gases. The Fb in dry air fell significantly just after the polarity reversal of the applied voltage and then gradually increased with the number of breakdowns. However, the Fb in SF/sub 6/ and N/sub 2/ retained nearly the same value after the polarity reversal. The Fb in the mixed gas of N/sub 2/ and O/sub 2/ fell after polarity reversal and decreased with increase of oxygen concentration. This is probably due to the fact that the ambient gas affects space-charge formation in PI thin films, Because it is a porous structure, the possibility of breakdown characteristics of ambient gas-PI thin film composite system may be taken into account.