Packing problems of sodium borate and boric acid powders are investigated. The apparent volume per gram of the samples in loosest packing is nearly constant in the region larger than the critical size, but in the region smaller than that, it increases gradually with decrease of particle diameters. The critical diameters obtained at the loosest packing in air are equal to that obtained by measurement of angles of repose and sliding angles. The relations between porosity (ε_o, ε_c) or the apparent density (ρ_c, ρ_o) and particle diameter (D) are represented by ε_o-ε_c=A exp (-BD) ρ_c-ρ_o=ρA exp (-BD) and the following relations have also been obtained between porosity or apparent density and C/Mg for sodium borate powders. ε_o-ε_c=0.43 C/Mg ρ_c-ρ_o=0.72 C/Mg (ε_o-ε_c) or (ρ_c-ρ_o) might be adopted as the degree of cohesion.