Heat generation density from semiconductor devices has been increasing with the rapid development of electronic technology. The cooling system using boiling two-phase phenomena has attracted much attention because of its high heat removal potential. To develop compact and high-performance cooling systems, we conducted experiments on the increase of critical heat flux (CHF) for flow boiling in narrow channels by improved liquid supply. A large surface of 150 mm in heated length and 30 mm in width with grooves of an apex angle of 90°, 0.5-mm depth, and 1 mm in pitch was employed. A structure of narrow heated channel between parallel plates with an unheated auxiliary channel was devised and tested by using water for different combinations of gap sizes and volumetric flow rates, where inlet of the main heated channel and the outlet of auxiliary unheated channel were closed to prevent the flow instability observed frequently at low flow rate for parallel two channels. For the total volumetric flow rate more than 4.5 × 10-5 m3/s, higher values of CHF large than 2 × 106 W/m2 were obtained for gap size of 2 mm. For gap sizes of 2 mm and 5 mm at high volumetric flow rate larger than 6.0 × 10-5 m3/s, or mass velocity based on the cross section are of main heated channel 958.1 kg/m2s and 383.2 kg/m2 s, respectively, the extension of dry patches was observed at the upstream location of the main heated channel resulting in burnout not at the downstream but at the upstream. By the increase in total volumetric flow rate, the pressure drop increased because of increasing in the flow rate passing through the sintered metal porous plates connecting both channels. The values of pressure drop for gap size of 2 mm were higher than that for gap size of 5 mm. When the performance of the cooling system was evaluated on the basis of pump power, ignoring its variation in the efficiency with volumetric flow rate, that is, the power defined as the product of the pressure drop and the total volumetric flow rate, higher values of CHF were obtained for gap size of 5 mm as far as the same pump power was concerned.