Junction　temperature　is　an　important　factor　affecting　the　reliabilities　of　semiconductor　devices.　Usually,　the　method　of　measuring　the　junction　temperature　is　not　tested　on-line.　However,　due　to　the　fact　that　neither　contact　thermal　resistance　nor　thermal　resistance　varying　with　temperature　is　taken　into　account,　there　exists　an　error　in　the　off-line　measurement.　A　way　to　solve　the　problem　of　off-line　measurement　is　to　measure　the　junction　temperature　on-line.　In　this　paper,　we　propose　an　electrical　method　of　measuring　the　temperature　rise　of　high-power　bipolar　transistor　in　the　working　condition.　The　measurement　method　is　based　on　a　good　linear　relationship　between　base-emitter　voltage　(V-be)　and　temperature　during　the　steady-state.　Taking　the　model　2N3055　of　bipolar　high　power　transistor　for　example,　in　this　paper　we　study　the　relationship　between　base-emitter　voltage　(V-be)　and　temperature　under　the　conditions　of　constant　collector-emitter　voltage　(V-ce)　and　collector-current　(I-ce).　During　the　experiment,　the　device　is　placed　in　a　thermostat.　A　voltage　is　applied　to　the　device　collector,　a　current　is　applied　to　the　base,　and　the　emitter　is　earthed.　Before　the　device　is　measured,　we　set　different　temperatures　and　make　sure　that　the　equipment　is　in　a　steady　state.　In　order　to　avoid　the　effect　of　self-heating,　the　pulse　current　is　used　in　the　experiment.　The　pulse　width　and　the　period　are　500　mu　s　and　1　ms,　respectively.　The　research　result　shows　that　the　base-emitter　voltage　(V-be)　decreases　linearly　with　temperature　increasing　and　the　base-emitter　current　is　changed　below　4%　when　the　temperature　is　in　a　range　of　40　degrees　C-140　degrees　C.　In　this　paper　we　also　deduce　the　mathematical　expressions　for　base-emitter　voltage　(V-be)　and　temperature　under　a　steady　state.　It　is　proved　that　the　V-be-temperatrue　curve　is　linear　and　temperature　error　is　less　than　0.5　degrees　C　when　the　changes　of　base　current　value　does　not　exceed　4%.　Therefore,　in　this　paper　we　deduce　a　new　method　of　testing　the　junction　temperature　in　the　speeding　up　measurement　experiment.　By　measuring　any　of　the　reference　points　on　the　calibration　curve　under　certain　experimental　conditions,　the　junction　temperature　can　be　calculated　quickly　according　to　the　proposed　formula.　Finally,　the　phase　11　is　used　to　verify　the　proposed　method.　We　measure　the　thermal　resistance　upper　the　case　for　the　junction　of　model　2N3055　and　the　thermal　resistance　under　the　case　for　the　junction　of　model　2SD1047.　The　measurement　results　of　phase11are　compared　with　the　junction　temperature　calculated　using　the　test　formula.　The　results　show　that　the　error　of　junction　temperature　between　the　two　methods　is　less　than　0.7%,　which　is　corresponding　with　the　needs　of　practical　application.　It　proves　the　correctness　and　feasibility　of　the　method.