Experiments　were　carried　out　to　investigate　forced　convective　heat　transfer　of　distilled　water　flowing　through　quartz　microtubes　with　inner　diameter　of　242　μm,　315　μm　and　520　μm.　The　microtube　was　heated　by　saturation　steam　to　keep　a　constant　temperature.　The　wall　average　temperature　was　obtained　according　to　saturation　pressure　of　stream　and　achieved　the　synchronization　of　heating　and　measuring　temperature.　As　the　Reynolds　number　varied　in　the　range　from　100　up　to　6000　and　the　Nusselt　numbers　were　measured,　correspondingly.　The　experimental　results　were　compared　with　the　classical　convective　heat　transfer　correlations　in　laminar,　transitional　and　turbulent　regimes　for　conventional　tubes.　The　comparison　results　indicated　that　the　Nusselt　number　may　be　less　than　the　classical　laminar　correlations　when　the　Reynolds　number　is　lower.　The　Nusselt　number　quickly　increased　with　increasing　of　the　Reynolds　number.　As　Reynolds　number　was　larger　than　1600-1900　and　3500-5000,　the　corresponding　Nusselt　number　will　reach　the　value　of　the　transitional　correlations　or　that　of　the　turbulent　correlations.