An　experimental　investigation　of　fluid　flow　friction　and　heat　transfer　coefficient　in　simultaneously　developing　flow　through　a　multiport　microchannel　flat　tube　(MMFT)　was　presented.　The　cross-sectional　geometries　of　five　tubes　were　rectangular　with　hydraulic　diameters　of　0.8-1.33mm　and　aspect　ratio　of　0.44-0.94.　The　working　fluid　was　water,　and　the　Reynolds　number　was　in　the　range　150-4500.　The　experiment　result　showed　that　friction　factor　was　successfully　predicted　by　classical　correlation　in　laminar　regime,　whereas　the　laminar-turbulent　transition　in　the　developing　flow　was　not　as　obvious　as　in　the　completely　developed　flow.　The　greater　aspect　ratio　produced　stronger　heat　transfer　capacity　in　the　developing　flow,　although　the　effect　of　the　aspect　ratio　decreased　at　increased　Reynolds　numbers　for　heat　transfer　characteristics.　Moreover,　the　scale　effect　improved　the　heat　transfer　performance　of　MMFTs,　especially　at　high　Reynolds　numbers.