A novel pilot scale microreactor in continuous re-entrance flow was evaluated in selective extraction of Co from Ni sulfate solution with Cyanex 272. The microreactor had three-dimensional (3D) conical sections symmetrically connected in 6 series with an internal volume of 12 ml and was capable of handling flow rate of 9 l/h. Using this extraction system correlation relating pressure drop and energy dissipation to flow rate was studied. Effects of variation of flow rate/residence time and extractant concentration on extraction and separation efficiency were investigated. The results indicated that high extraction ratio of Co, good selectivity for separation of Co from Ni at low Cyanex 272 concentration were obtained in single-stage microextraction system. Above 99.9% of Co was separated from aqueous phase in two-stage counter-current extraction processes. Moreover, mass transfer performance between the two immiscible phases was investigated and compared to other unstructured microchannels. The overall volumetric mass transfer coefficient k(L)alpha of the target Co in the special 3D conical microstructure were superior to other simple microchannels due to the high extraction ratio at short residence time caused by re-entrance flow effect.