A radioactive particle tracking technique is used to measure classification velocity of a radioactive tracer falling in a liquid-solid fluidized bed. The liquid passing through the fluidized bed is water and nine different types of particles are used as fluidized particles. A radioactive particle is dropped into the fluidized bed in order to measure its falling velocity. Emitted gamma-rays from the radioactive tracer are detected by eight NaI scintillation detectors placed around the bed. The number of counts of gamma-rays are then used to calculate the coordinates of the tracer from which the velocity of the tracer is calculated. The classification velocity is used to calculate the effective drag coefficient. The correction factor, f, which is defined as the ratio of effective drag coefficient over standard drag coefficient at the same slip velocity, is then evaluated. This factor is a strong function of the porosity and may be expressed as f = epsilon(-m), in which epsilon is the porosity of the bed. The exponent m, is well known to be dependent on terminal Reynolds number of the falling particle. It is found from the present study that it is also a function of Archimedes number of the fluidized particles. The data from this work and some other data in the literature are used to develop a correlation for calculating the correction factor as a function of terminal Reynolds number of the falling particle, Archimedes number of the fluidized particles and a geometric parameter which is the ratio of diameters of the falling particle to that of the fluidized particles.