Comminution energy efficiency of biomass particle production and particle physical properties are important variables that directly influence feedstock supply-conversion chain from the on-farm production to the biorefinery. In this study, size reduction and particle physical property analysis of miscanthus (Miscanthus giganteus), switch grass (Panicum virgatum), weeping willow (Salix babylonica), and energy cane (Saccharum spp.) were conducted from the perspectives of feedstock supply and conversion, by using commercial-scale David Bradley chopping machine, bench-scale Retsch SM2000 knife and SK100 hammer mills. Results indicated that comminution energy consumptions were exponentially correlated with aperture sizes of the mill screens for biomass crops. Moisture contents significantly influenced comminution energy consumption, especially for finer size reduction. Given a specific mill screen, Retsch SK100 hammer mill was more efficient than SM2000 knife mill which mainly attributed to higher motor speed and material axial feeding. While particle size was negatively related to bulk densities of biomass chops, comminution ratio of biomass size reductions were positively proportional to energy consumptions for all of the four biomass crops. Bulk densities for 4-mm and smaller miscanthus and switchgrass chops were higher than that of bale. For willow and energy cane, grinding did not increase but reduce bulk densities. Particle size and surface area estimates from the commonly-used ANSI/ASAE Standards S424.1 and 319.4 were highly sensitive to particle size distributions. Further studies on standardization of particle size and surface area estimates are needed.