Hydrodynamic transition experiments for two-phase (liquid-solid), both upward and downward, liquid flow systems were performed in a 127-mm diameter column. The particles were 3.2-mm polymer (1,280 kg/m(3)), 5.8-mm polyethylene (910, 930, 946 kg/m(3)), 5.5-mm polystyrene (1,021 kg/m(3)) and 6.0-mm glass (2,230 kg/m(3)) spheres, with water, aqueous glycerol solution and silicone oil as liquids. The dimensionless pressure gradient increases initially with increasing liquid velocity, but decreases gradually with increasing liquid velocity beyond Ulmf due to bed expansion. The non-dimensionalized pressure gradient using the liquid/solid mixture density increases with increasing liquid velocity and then reaches a constant value close to unity beyond U(lmf). The minimum fluidization Reynolds number for liquid-solid system increases with increasing Archimedes number including both heavier and lighter than the density of the liquid phase. Ulmf should be the same for both upward and downward fluidization systems since the Ergun equation is based on the main assumption that drag force of the superficial liquid velocity, Ulmf, is equal to the net difference between gravitational and buoyancy forces.