A simple microtube heater for supercritical state water or solutions that call operate at up to 873 K and 25-50 MPa has been developed. In this study, the heating process in a Joule heating microtube and the heat transfer inside the microchannel were numerically investigated by a Coupled analysis using simplified modeling. The modeled microtube was made of Ni-base alloy 625; its OD was 1.59 mm and the 1D of the microchannel was 0.25-0.50 mm. ne typical length of the microtube was 200-250 mm. The characteristics of the heal-up process at the microtube wall and the microchannel heat transfer to flowing water were determined through numerical simulations. As a practical low-cost method, it combination of AC voltage charging and a power thyristor was adopted for heater control. It was observed that if the current frequency was over 50 Hz, the fluctuation in the outflow temperature remained below 1.5 K. The heating performance of the Joule-heating microtube heater is restricted by the pressure drop in the microchannel and the maximum temperature for the tube material.