Choosing　small　and　medium　power　switching　transistors　of　the　NPN　type　in　a　3DK　set　as　the　study　object,　the　test　of　accelerating　life　is　conducted　in　constant　temperature　and　humidity,　and　then　the　data　are　statistically　analyzed　with　software　developed　by　ourselves.　According　to　degradations　of　such　sensitive　parameters　as　the　reverse　leakage　current　of　transistors,　the　lifetime　order　of　transistors　is　about　more　than　104　at　100°C　and　100%　relative　humidity　(RH)　conditions.　By　corrosion　fracture　of　transistor　outer　leads　and　other　failure　modes,　with　the　failure　truncated　testing,　the　average　lifetime　rank　of　transistors　in　different　distributions　is　extrapolated　about　103.　Failure　mechanism　analyses　of　degradation　of　electrical　parameters,　outer　lead　fracture　and　other　reasons　that　affect　transistor　lifetime　are　conducted.　The　findings　show　that　the　impact　of　external　stress　of　outer　leads　on　transistor　reliability　is　more　serious　than　that　of　parameter　degradation.　©　2013　Chinese　Institute　of　Electronics.