The scale-up of microstructured reactor is an essential issue in order to achieve required throughput for industrial application. A high speed camera was used to observe gas-liquid flow pattern and phase distribution in four parallel microchannels. The influence of flow rate and viscosity on flow uniformity was investigated. The experimental liquid phase was deionized water with 0.3%(mass) surfactant sodium dodecyl sulfate (SDS)-glycerol

and the gas phase was nitrogen (N2). Six types of two-phase flow patterns were observed in the parallel microchannels. For the situation of all slug flow in four branch channels

the distribution of bubble length and velocity was studied. At a given gas flow rate

the relative standard deviation (RSD) of bubble length in branch channels increased with increasing liquid flow rate

and the RSD of bubble velocity in branch channels increased with increasing liquid flow rate up to a maximum and then gradually decreased. Non-uniformity of gas phase distribution increased with the increase of liquid flow rate and viscosity

and non-uniformity of liquid phase distribution decreased with the increase of liquid viscosity. The influence of gas flow rate on two-phase distribution was not significant. The study is helpful for the design and optimization of parallel microchannel structure to realize the uniform two-phase distribution.