'Centrifuge-on-a-chip' is a versatile and multifunctional microfluidic chip which can conduct operations of selective trapping, enrichment, labeling and solution exchange. In this work, the mechanism of selective trapping ability was studied by both the experiments and numerical simulation. We proved the existence of shear gradient lift force in the expansion-contraction cavity and derived an explicit expression of it. Also, a numerical modeling was developed to study the trapping process of micro-particles in a 'Centrifuge-on-a-chip'. The effects of flow condition and particle size on the trapping process were investigated. It demonstrates that the derived expression and developed model are suitable for the prediction of particles' trapping process, which can assist the design of 'Centrifuge-on-a-chip'. © 2013 IEEE.