In the present study, the characteristics of gas-liquid Taylor flow in a fractal microchannel network were investigated by numerical modeling and experiments. With perpendicular gas intake, the fractal design methodology resulted in uniform flow distribution. Unstable Taylor flow and saddle-shaped velocity profile were observed in a high-level branch with 0.3 mm width. The liquid slug exhibited different flow fields during splitting in the bifurcations of different levels. The significant size effects were further investigated in four channels representing different sizes (0.3, 0.6, 1.2, and 2.4 mm) via a μ-PIV and high-speed camera. The normalized slug length (LS/W) increased with the increasing gas-liquid flow rate ratio (jG/jL), while the bubble length (LB/W) followed an opposite trend. The stability of the gas-liquid Taylor flow pattern became worse when the gas velocity increased beyond a certain value (Reynolds number Re > 220) under the microscale effect. Two typical counter rotating vortices were observed in the liquid slug, and the swirling strength increased exponentially with the jG/jL. It was found that both the velocity profile in the 0.3 mm channel deviated from the laminar flow. Particularly, a flow field similar to turbulence was observed in the 0.3 mm channel when the jG/jL reached 4. This work quantified the significant size effect in micro- and millichannels, providing theoretical basis for the effective scale-up of the microreactor.