We present an experimental study of the liquid-liquid system with extremely low viscosity ratio (10−3) in a T-shaped microchannel with 120 × 120 μm inlets and a 240 × 120 μm outlet channel. Six different flow patterns have been observed: plug, droplet, slug, throat-annular and parallel flow. A specific plug flow pattern was found where micron-sized droplets or even a jet breaks off from the rear meniscus of a plug. In addition to typical Taylor-shaped plugs the dumbbell-like plugs were observed. Flow visualization data were summarized in the flow pattern maps. Plug length and velocity were measured based on flow visualization results. It was found that the plug velocity can be fitted by power function rather than the linear function, which disagrees with experimental data at a low plug velocity. Front and tail plug surface curvature was scaled using dimensionless parameter, the flow rate ratio multiplied by Capillary number based on bulk velocity (Qd/Qc * Cabulk). Instantaneous velocity vector fields inside water plugs were measured by means of PTV technique. Different flow structures were found and discussed. As a result, it is proposed to use the values of Qd/Qc * Cabulk for distinguishing different plug shapes and circulation patterns inside the plugs.