Biomass feedstock in general and herbaceous feedstock in particular, is characterized by a low bulk density, low energy density and high variability in chemical and physical composition. Mechanical pre-processing such as size reduction and densification can significantly improve feedstock properties. To commercialize feedstock production on a large scale, there is a need for precise, instantaneous methods allowing the measurement of material properties. The objective of this study was to investigate the relationship between biomass electrical capacitance and bulk density during mechanical densification. A lab-scale hydraulic compressor was built and employed by combining two lateral and two vertically oriented capacitor plates into a mini-bale compression chamber. A MatLab® data acquisition system was developed to record pressure, plunger position and biomass capacitance simultaneously. The experimental design included three feedstock types (Miscanthus, switch grass and sugarcane bagasse), one particle size (ground through a 6.35-mm screen by a hammer mill) and three moisture levels (air-dry (oven-dried at 49°C for 72 h), 13% and 17.5% of moisture content). The results showed significant relationships between biomass capacitance and bulk density during compression. Biomass moisture significantly increased biomass capacitance but did not affect the positive relationships between bulk density and capacitance. The study showed that electrical capacitance may serve as a proxy measure for biomass density.