Both Newtonian fluid flow and power-law non-Newtonian fluid flow through porous media composed of single-diameter and multi-diameter spheres were studied experimentally. The pressure drop characteristics for the packed beds with single-diameter spheres were validated well by theoretical models. For the packed beds containing spheres with two different sizes and three different sizes, three types of mean diameters were examined in the theoretical model, and the mass mean diameter is recommended as the effective diameter based on the present experimental data.