A　two-dimension　thermal　model　is　presented　for　laser　diode　bars　and　the　transient　temperature　distribution　is　simulated　and　discussed.　It　is　found　that　the　transients　display　three　time　ranges　with　different　temperature　slopes:　in　the　range　before　some　ten　microseconds,　the　temperature　difference　is　small　between　the　bars　with　different　fill　factors;　in　the　range　between　some　ten　microseconds　and　some　ten　to　hundred　milliseconds,　the　temperature　of　bars　with　high　fill　factor　increases　rapidly;　at　some　ten　milliseconds,　a　steady-state　thermal　distribution　is　nearly　reached　for　the　bars　bonded　on　the　micro-channel　heat　sink.　While　a　steady-state　thermal　distribution　is　reached　at　some　hundred　milliseconds　for　the　bars　bonded　on　the　heat　conducting　sink.　Temperature　difference　between　the　emitters　appears　earlier　for　the　bars　with　high　fill　factor　at　the　same　injecting　current.