Experiments were made to clarify the mean void fraction, the mean holdup, the velocity of liquid slug and the flow patterns of air-water two-phase flow in horizontal flat rectangular microchannels with the dimensions of the width and depths of 1.0 x 0.1 mm and 1.0 x 0.2 mm, respectively. The flow patterns of bubble flow, slug flow and annular flow were observed. These data in the microchannel showed relatively similar tendencies to that in the minichannels with the width of 1~10mm and the depth of 1mm which we had previously reported. In a channel of 1.0 x 0.1 mm, however, the mean holdup and the holdup which correspond to the base film thickness in annular flow gave larger values because the effects of liquid viscosity and surface tension on the holdup and void fraction became dominant. The remarkable flow characteristics of rivulet flow and the flow with partial dry out mainly on the upper channel wall were observed in slug flow and annular flow regions in the channel of 0.1 mm depth.