This　paper　utilized　an　IR　camera　to　measure　the　temperature　of　outer　wall　of　the　micro　steel　tubes　with　inner　diameter　of　168　μm　and　399　μm,　and　inner　roughness　of　8%　and　9%　respectively,　through　which　distilled　water　flowed　as　the　working　fluid,　under　conditions　of　constant　heating　power　and　variant　Reynolds　number　(Re).　It　also　measured　the　pressure　drop　and　the　flux　at　the　inlet　and　outlet　of　the　micro　steel　tube,　and　calculated　the　Nusselt　number　(Nu)　and　frictional　resistance　coefficient　(f)　under　different　Re.　The　results　show　that　when　Re　is　lower,　f　is　in　rough　agreement　with　Hagen-Poiseuille　solution,　however,　Nu　is　slightly　higher　than　Hausen　solution.　When　Re　reaches　at　800,　f　begins　to　depart　from　Hagen-Poiseuille　solution,　and　Nu　begins　to　increase　quickly,　which　is　due　to　earlier　transition　from　laminar　flow　to　turbulent　flow.