Biomass transportation is hampered by the inherent low bulk density of herbaceous materials, causing containing equipment to reach their volume limit far before their weight limit. To evaluate the purported efficiency increase, it is imperative to determine the energy requirement of the compression process. In addition, to design containers for compressed biomass transportation, mechanical parameters of the biomass, such as Poisson's ratio, must be determined. This paper describes the measurement of these quantities for Miscanthus giganteus and switchgrass under uni-axial compression, using four particle sizes ranging from unground (about10-30 cm) to 25.4mm, 12.7mm, and 6.35mm, at three repetitions each. Tests were conducted using an instrumented machine capable of generating a compressive force of 890kN, which, using a biomass containing cylinder with an inner diameter of 396mm, exerted a maximum pressure of 7.2MPa on the biomass. The energy requirement for compression of Miscanthus giganteus and switchgrass, expressed in a Percentage of the Inherent Heating Value (PIHV) of the materials, ranged from 0.01 to 0.047 PIHV. These low values indicate that the energy requirement of compression is not prohibitive, but rather, that the maximum density attainable is limited by the size and costs of the machinery required. For Miscanthus giganteus, the energy requirement among the four particle sizes, ordered from highest to lowest was unground, 12.7mm, 6.35mm and 25.4 mm. For switchgrass, the energy requirement ordered from highest to lowest was unground, 12.7mm, 25.4mm and 6.35 mm. These results indicate that the compression process of biomass is not easily understood using intuition.