Experimental investigations were carried out on a pilot scale slurry transportation apparatus to investigate wall-slip phenomenon and rheological properties of concentrated coal water slurries(CWSs) in pipe flows. Straight pipes with different inner diameters (25 mm, 32 mm and 50 mm) were selected to characterize the flow behaviors of CWSs with various solid contents (65.3 wt%, 67.1 wt% and 68.2 wt%). A procedure based on a new slip model was developed to determine the true theological behavior (independent from pipe diameter) and the slip velocity at the pipe wall. The results suggested that 65.3 wt% CWSs exhibited a Newtonian fluid behavior and the slurry flow was free from wall-slip effects. But at 67.1 wt% and 68.2 wt% solid contents, the slurry flows were strongly affected by wall slip and with the increase of wall shear stress. the slurries exhibited their true rheological behaviors firstly as a shear-thinning fluid and then as a shear thickening fluid. The existence of a minimum value of slippage contribution was indicative of the transition of flow behavior from shear thinning to shear thickening. (C) 2009 Elsevier B.V. All rights reserved.