The development of a simple and feasible fluid-solid separation device is critical to further advancement in the use of micro-technology. The mini-hydrocyclone, which possesses a concise geometry and simple operational process, has been proposed as a promising solution to bridge this gap since the cut-size decreases with decreasing hydrocyclone diameter. In this work, we investigated the fluid flow and particle separation ability of a 5 mm diameter mini-hydrocyclone through computational fluid dynamics (CFD) modelling. Direct numerical simulation (DNS) results with CFD have shown that the flow transition and subsequent unsteady state behaviour occurred in the mini-hydrocyclone at a low Reynolds number (Re-in = 300) because of the onset of centrifugal instability. The centrifugal instability offered an insight into the flow transition and the development of turbulent flow in hydrocyclones which have not been studied. The centrifugal instability in the mini-hydrocyclone begins as Gortler vortices developing in the boundary layer and they subsequently affect the flow field. Particle motion tracing showed that improved separation with finer cut size, d(50), and steeper separation sharpness were obtained as the inlet velocity was increased. The improvement can be explained by the flow characteristics when the flow transits to turbulent flow. (C) 2012 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.