Abstract: Hydrocyclone has been widely used for coal slime classification ever since its invention. However, there is a limitation to hydrocyclone accuracy: even though much work has been done to improve its structural and operational parameters, there are still misplaced particles in underflow and overflow. It’s shown that particle density has a role in sizing effect by analyzing terminal velocity, equal settling ratio and density. This can be thought of as separation side effect and some suggestions about how to improve classification efficiency was be put forward.