The paper presents an experimental study on the drag forces of two interactive particles with unequal diameters. A circulating loop for the fluid flow is set up. The particles are arranged in a line parallel to the direction of the flow in the test section. Direct measurements of the drag force of the downstream trailing particle are performed by utilizing a micro-balance force measuring system. The tests are conducted under different conditions. The obtained results illustrate the effects of particle size ratio and Reynolds number on the drag force of the trailing particle. It is found that the drag force ratio of the trailing particle increases with the particle size ratio under both circumstances of the particle size ratio being larger than 1 and less than 1. The drag force ratio of the trailing particle tends to decreases with increasing particle Reynolds number for the present cases being studied.