In　this　work,　different　sizes　of　micro-light-emitting　diodes　(mu　LEDs)　were　fabricated　on　the　sapphire　and　GaN　substrates.　The　thermal　characteristics　of　mu　LEDs　were　studied　by　the　forward　voltage　method,　thermal　transient　measurement,　and　infrared　(IR)　thermal　imaging.　The　mu　LEDs　on　the　GaN　substrate　showed　an　approximately　10　degrees　C　lower　junction　temperature　and　smaller　amplitude　of　the　K　factors　than　those　on　the　sapphire　substrate　under　the　current　injection　level　of　4　kA/cm(2).　IR　thermal　imaging　results　showed　the　uniform　temperature　distributed　on　the　GaN　substrate.　The　thermal　transient　measurement　showed　that　the　thermal　resistances　of　the　mesa,　epilayer,　and　the　interface　of　GaN/substrate　were　reduced　significantly　for　mu　LEDs　on　the　GaN　substrate.　This　means　that　a　high-quality　GaN　crystal　and　homogeneous　interface　corresponded　to　little　scattering　for　phonons.　The　APSYS　simulation　indicated　that　the　high　thermal　and　electrical　conductivity　of　the　GaN　substrate　played　a　key　role　in　the　low　junction　temperature　and　uniform　temperature　distribution.　A　small-sized　mu　LED　combined　with　a　GaN　substrate　can　become　a　perfect　candidate　for　high-power　applications　and　visible　light　communication.