This study performs experiments to examine the single-phase heat transfer and pressure drop in small-diameter microfin tubes 2.5–4 mm outer diameter using water as the test fluid.The geometric parameters of the test microfin tubes are as follows: 25 fins,7–16° helix angle,and 0.1mm fin height.For the Reynolds number ranging from 500 to 10,000,the pressure drop and the average heat transfer coefficient are measured under adiabatic and constant heating conditions,respectively.