An investigation of the power input required to suspend a high concentration of solid particles was carried out in a draft-tube stirred vessel. The relation between the "just complete suspension" rotational speed of an impeller and the power consumption was measured under various important operating conditions. It was found that the power consumption at just complete suspension was minimized when the ratio of the inner cross-sectional area or the draft tube to the outer cross-sectional area or the annulus was 0.6 when the discharge flow was in the down direction. In this case, the just complete suspension of a glass bead slurry of 600 mu m particles at 40 vol% was achieved with low power consumption.