Flow boiling heat transfer in small-sized channels employed in compact heat exchangers is attracting increasing attention with the progressing miniaturization in many industrial fields. Experiments have been conducted to investigate the heat transfer and the pressure drop in narrow channels (rectangular channels with 2×4 mm and 0.5×2 mm (width × depth)). The working fluid is deionized water. The flow rate is 100 up to 700 kg/m2s at exit atmospheric pressure. By means of video-camera, flow boiling phenomena were observed as well. Three flow patterns, i.e. bubbly flow, slug flow and annular flow, could be distinguished. Based on the present single-phase flow experimental results the flow boiling pressure drop was compared with the Lockhart-Martinelli correlation, and ±30% deviation from Lockhart-Martinelli was found.