The slit-like microchannels with large aspect ratio were carefully investigated for the demands on mass production and reliable operation of microreactors. The system of n-octane/SDS aqueous solution was used for observing the liquid-liquid microflows in PMMA slit-like microchannels with four different configurations. At the mode of cross-flow, it is easy to form jetting flow and laminar flow and the threshold for flow patterns transition lies in Red about 200. New flow patterns like double jetting or laminar flow and fiercely dispersed flow can be achieved at the mode of perpendicular flow. Mass transfer performance in the slit-like microchannel was also studied with n-butanol/succinic acid/water as a working system. The influences of the flow rates and two-phase flow ratio on mass transfer performance were determined. With both modes of dispersion, Murphree efficiency above 95% could be achieved within 1 s at high flow rate. Using the Re of the continuous phase, we set up the phenomenological relation between flow patterns and mass transfer performance that the mass transfer rate increases sharply after Re, reaching around 450 where the flow pattern change into fiercely dispersed flow at the mode of perpendicular flow. With the same Murphree efficience, the throughput of a slit-like device is comparable to a microchannel array with 300 channels, rendering this device attractive for potential industrial application. (C) 2014 Elsevier BM. All rights reserved.