Flow and Heat transfer characteristics of the boiling two-phase flow of water in flat mini-rectangular-channels were examined. The cross-section was 0.5x10 or 0.2x10mm and the length was 250mm including 50mm heating length. When the heat flux was low, the heat transfer coefficient agreed well with the value of the Dittus-Boelter's correlation. The onset condition of boiling was well predicted by the Bergles-Rohsenow's correlation. After the boiling was initiated, the heat flux was lower than values of Rohsenow saturated-pool boiling correlation. This tendency became more prominent in the 0.2mm gap case. The pressure drop of the subcooled flow and the transition condition from laminar flow to turbulent flow were well expressed with the correlations for the conventional size. The boiling two-phase flow pressure drop was lower than the prediction by the correlations for the conventional size and close to the correlation for a mini tube.
