The heat transfer performance of a microtube was experimentally investigated to verify its suitability as a temperature control device for supercritical water (SCW) reactions under high-pressure and high-temperature conditions (pressure: 23-45 MPa, temperature: 673-773 K, flow rate: 0.6-4.6 kg/h). The microtube was made of Ni-base alloy 625 and had a microchannel with an internal diameter (ID) of 0.258 mm. Ohmic heating was employed for elevating the temperature of the microtube. Using purified water as the operating fluid for thermal performance evaluation, rapid and effective fluid heating from room temperature to 673 K within 0.05 s was achieved inside the heater and heat exchanger, respectively. The electrical power input was converted to fluid enthalpy by the microtube heater with a thermal efficiency of 75-95%, heating rate of 10(4)-10(5) K/s, and an overall heat transfer coefficient of 4950-35400 W/(m(2) K). Experimental results indicate that the microtube is useful as a temperature control device, and the rapid and effective turbulent heat and mass transfer in the microchannel is suitable for the high-speed temperature control required for SCW reactions.