A　model　of　Heat　source　is　presented　for　tunnel　regeneration　semiconductor　laser　diodes.　Three-dimensional　transient　and　steady-state　temperature-distribution　of　the　laser　diode　with　two　active　regions　is　simulated　by　using　the　finite　element　method.　It　is　found　that　the　temperature　increase　of　the　active　regions　is　small　during　the　initial　several　microseconds.　In　the　range　between　several　microseconds　and　some　ten　milliseconds　the　temperature　of　the　active　regions　increases　significantly.　At　some　hundred　milliseconds　a　steady-state　thermal　distribution　is　nearly　reached.　It　is　in　agreement　with　the　measured　data.　The　temperature　of　two　active　regions　is　the　highest　initially,　but　the　temperature　of　the　tunnel　junction　increase　rapidly　and　is　higher　than　the　temperature　of　the　active　region　close　to　the　heat　sink　finally.　The　temperature　of　the　rear　facet　increases　much　slowly　for　the　asymmetric　packaging　along　the　resonator.　Steady-state　temperature　distribution　present　that　temperature　of　the　active　region　close　to　the　substrate　is　higher　than　that　of　the　active　region　close　to　the　heat　sink,　but　the　temperature　difference　is　less　than　0.3K.　The　highest　temperature　is　in　the　stripe　center　of　the　front　emitting　facet　of　the　active　region　close　to　the　substrate.