Nitrogen　gas,　used　as　a　working　fluid,　flows　through　a　micro　steel　tube　with　inner　diameter　168　μm　and　outer　diameter　406　μm.　Using　directly　electrical　heating　method,　the　outer　wall　temperature　fields　of　the　micro　steel　tube　are　acquired　by　employing　an　IR　Camera　and　specially　magnifying　lens　for　constant　flow　rate,　different　heating　power　and　constant　heating　power,　different　flow　rate,　respectively.　The　flux　and　the　temperature　of　the　inlet　and　the　outlet　of　the　micro　steel　tube　are　measured　synchronously.　A　correlation　between　the　axial　conductive　heat　and　the　convective　heat　transfer　is　obtained　by　a　theoretical　analysis　based　on　the　experimental　results.　Investigations　clearly　show　that　the　axial　conductive　heat　weakens　the　convective　heat　transfer　in　micro　steel　tube　and　the　decrement　may　be　more　than　2%　compared　to　the　quantity　of　whole　convective　heat　transfer　when　the　working　fluid　is　nitrogen　gas.