A　new　method　for　calculating　low　probability　impulse　breakdown　voltages　is　put　forward,　which　consists　of　three　limiting　lines:　the　streamer　breakdown　line,　the　low　probability　impulse　breakdown　level　and　the　streamer　corona　onset　line　in　different　gas　pressure　ranges.　The　streamer　breakdown　line　and　the　streamer　corona　onset　line　can　be　calculated　using　the　basic　knowledge　of　SF,　gas　insulation.　If　the　critical　pressure　P-c　is　known,　then　the　low　probability　impulse　breakdown　level　can　be　predicted,　because　the　streamer　corona　onset　voltage　and　the　low　probability　impulse　breakdown　voltage　are　of　almost　the　same　value　at　P-c.　So　the　expression　of　P-c　is　very　useful　for　insulation　design.　In　this　paper,　different　point/plane　gaps　are　tested,　and　an　expression　of　P-c　is　derived　based　on　the　experimental　results.　Compared　with　the　measured　values　for　given　gaps,　the　calculated　values　have　a　relative　error　no　more　than　2%.