The microchannel is one of the essential components for the construction of various micro systems. However, it has been reported that the flow and heat transfer behavior in a microchannel deviates from predictions based on the conventional assumption generally accepted in macro scale channels. Frictional characteristics of nitrogen and helium flows in the three different microchannels (hydraulic diameter ranging 3 to 10µm) have been investigated experimentally. The frictional resistance of gaseous flow in trapezoidal cross-section microchannel was observed to be smaller than that in the conventional sized channel. The reduced frictional resistance in microchannels is caused by the rarefaction effect due to extremely small dimensions of flow passages. By using Maxwell's first order slip boundary condition, we can well predict the mass-flow rate through microchannels and the friction constant.