Horizontal two-phase flows within rectangular channels with small gap heights have been examined experimentally. The gap heights range from 0.4mm to 4mm corresponding to aspect ratios(the channel height divided by the width) from 0.02 to 0.2 Water and air were used as the test fluids with the superficial-velocity ranges being 0.03-2.66m/s and 0.05-18.7m/s, respectively. The region of each flow pattern has been graphically represented in the Mandhane-type map. With decreasing of the gap-height, the plug-flow region increases while the bubbly-flow region shrinks down. On the other hand, the criteria of the plug-slug transition and the slug-annular transition are little affected by the change of the gap height. In addition, the correlations representing the plug-bubbly and plug/slug-bubbly transitions are proposed.