An experimental investigation of subcooled flow boiling in a rectangular mini-gap channel with the dimension of 0.5 mm × 5 mm was conducted with deionized water as the working fluid. Fabricated by electroless plating method and high-temperature treatment, the copper-based hydrophobic micro-porous surface was utilized in the experiments. High-speed flow visualization was conducted to picture the flow patterns during the experiment. The mass fluxes were in the range of 200–400 kg/m2s, and the wall heat fluxes were spanned from 35 to 350 kW/m2. The onset of flow boiling, heat transfer coefficient, and pressure drop were discussed with the variation of heat fluxes and mass fluxes, the trends of which were analyzed along with the flow patterns. Because of the numerous nucleation sites on micro-porous surface, the superheat required for the onset of boiling are of small amounts of about 2 K. Due to the intense nucleation process, the boiling curves appeared to be a negative slope after the onset of boiling, which was more obvious in the lower mass flux conditions. In the high heat flux conditions, heat transfer coefficients under lower mass flux condition were higher because the intense nucleation process occurred and the elongated bubble flow along with the film evaporation heat transfer was formed. The film evaporation heat transfer inside the elongated bubble is more efficient to release the latent heat than the nucleate boiling. However, the appearance of the elongated bubble flow would attribute to higher pressure drop and severer pressure drop fluctuation due to its expansion toward upstream.