An experimental investigation is presented of the effect of the particle size distribution on the compaction of cohesionless powders. It is demonstrated that the densification of a powder during compaction may be treated as a (pseudo-) rate process, and the porosity-pressure relation can be described by a simple and general equation. The results indicate that particle size distribution plays an important role in quantifying the porosity-pressure relation, though this relation may be dominated by the mechanical properties of powders at high pressures. A possible method of predicting the relationship between porosity and particle size distribution and pressure is also discussed.