Water-butanol and water-hexane flows were visualized in ultra-shallow straight and serpentine microchannels with a crossjunction. At the inlet cross-junction, three major flow patterns including tubing/threading, dripping and jetting were mapped using the aqueous Capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular flow, slug flow and droplet flow were mapped using combined dimensionless numbers (Weber number times Ohnesorge number) of both phases. The flow pattern transitions were explained based on a force analysis, considering the phase flow rates, junction angle between the side feeding channels and the central feeding channel as well as aspect ratios. Compared to the straight microchannel, the dripping regime at the inlet junction and the slug flow occupy larger zones in serpentine microchannels because the centrifugal force tends to break up the organic annular core into slugs and droplets over the bends. Nomenclature Bo Bond number, (ρ a-ρ o)gd h 2 /γ Ca Capillary number, μu/γ Ce A dimensionless number representing the ratio of centrifugal to interfacial forces, ρu 2 d h 2 /(γR) d Channel diameter, μm d h Hydraulic diameter, μm g Gravitational acceleration, m/s 2 h Channel depth, μm j Superficial velocity, m/s L Channel length, m Oh Ohnesorge number, μ/(d h ργ) 0.5 Q Volumetric flow rate, ml/h q Flow rate ratio of organic to aqueous phases R Bending curvature radius of the channel centerline, m Re Reynolds number, ρjd h /μ u Average velocity (the overall volumetric flow rate of the two phases divided by the cross-sectional area), m/s w Channel width, μm We Weber number, ρj 2 d h /γ Greek symbols γ Interfacial tension, N/m μ Dynamic viscosity, Pa•s ρ Density, kg/m 3 Subscript ave Average a Aqueous phase c Continuous phase d Dispersed phase o Organic phase