Convective heat transfer in microtubes has attracted considerable attention for almost three decades. It is well-known that the solid liquid interfacial tension plays an important role at the micrometer length scale. In this work, the characteristics of convective heat transfer in microtubes with inner diameters ranging from 0.353 to 1.045 mm were investigated; the effects of interfacial tension on heat transfer were also investigated using nine working fluids (including deionized water, 5 wt % ethanol aqueous solution, 50 wt % ethanol aqueous solution, ethanol, ethyl acetate, cyclohexane, n-hexane, cyclohexanone, and cyclopentanone). The experimental results showed that the performance of convective heat transfer of different working fluids in microtubes varied significantly and was different from that in conventional scale tubes. The results were correlated by an empirical correlation, in which the effect of solid liquid interfacial tension was taken into account. The agreement between the correlation and the experimental data was found to be satisfactory.