An experimental study was carried out to clarify the effect of viscosity on the flow characteristics of solid-liquid mixtures flowing through a vertical pipe. Spherical glass beads (0.19-2.2mm in diameter) were suspended in upward flows of aqueous solutions of glycerol (2.97 X 10(-3)-2.51 X 10(-2) Pa (.) s in viscosity) or water (1.14 X 10(-3) Pa (.) s in viscosity), and pressure drops were measured under various operating conditions. As a result, the solid-liquid mixture flow was found to show the transition from laminar to turbulent flow at lower velocity than the critical velocity for single-phase flow. This fact was confirmed by experiments of flow visualization and by pressure fluctuation measurement. A plot of friction factor vs. Reynolds number was examined to express the effect of liquid viscosity on pressure drop of the mixture flow. A good correlation between these parameters was obtained by adopting the superficial velocity of the liquid as the characteristic velocity. Critical Reynolds numbers for the flow of mixtures with different concentrations and sizes of particles were extracted from the plots and expressed by a simple empirical equation. Moreover, correlation equations of friction factor were proposed separately for laminar and turbulent flow regions.