Mill tailing are one of the most important components of cemented paste backfill (CPB). However, there is still a lack of knowledge on the effect that tailings fineness and density has on the quality of paste backfill. Therefore, this paper presents the results of an experimental study done to evaluate the influence of the tailings particle size and density on the quality of CPB. The criteria examined of the paste backfill quality are: its mechanical strength (uniaxial compressible strength), economic performance (based on binder cost), water demand (pulp density and water to cement ratio) for a given consistence and its microstructure (porosity and pore size distribution). The end results have shown that the tailings fineness and density significantly influence the properties of the hardened (strength, cost, microstructure) and fresh (water demand) paste backfill. (C) 2004 Elsevier Ltd. All rights reserved.