The flow distribution and mass transfer for gas-liquid flow in two parallel microchannels with different tree-shaped distributors are studied. Two parallel microchannels, both consisting of four parallel branches, were operated in the slug flow and with the Reynolds number in the range of 0.2-10. The operating range in the halving-width microchannel (Tree-1) is significantly narrower than that in the constant-width microchannel (Tree-2). As the ratio of the gas-liquid flow rate increases, the flow distribution in Tree-1 gradually deteriorates. However, in Tree-2, there exists a critical ratio of the gas-liquid flow rate to attain an ideal flow distribution. Meanwhile, the volumetric mass-transfer coefficient in Tree-1 is lower, while in Tree-2, the opposite is true. However, the high mass-transfer coefficient is always accompanied by a high pressure drop. The pressure drop in Tree-2 is higher than that in Tree-1. Furthermore, the performance ratio is adopted to evaluate the mass-transfer performance.