Understanding the physical properties of the high yielding miscanthus can help guide its engineered interactions with feedstock supply chain system unit operations and biomass conversion technologies. An important consideration in the design and selection of a grinding system for dry-herbaceous biomass is the required feedstock particle size and distribution.  Typically, the mean feedstock particle-size requirement for conversion to ethanol is 1/4 inch minus, which directly impacts the preprocessing system costs.  Field tests using a fixed cutter tub grinder provided data to compare the mean particle size and particle distribution for miscanthus feedstock using multiple grinder screen configurations. Analysis of the data involved using a forage particle separator (ANSI/ASAE Standards, 2001) to isolate the different sized particles from each grinder fraction.  Significantly, 45% of the material was below 0.25 inch in size for all grinder screen sizes except the no screen configuration. This fraction of the material is far below the nominal screen size of the grinder, indicating a tendency to reduce material to sizes smaller than dictated by the grinding screens.  Also, feedstock fraction performance in terms of chemical composition and simultaneous saccharification fermentation (SSF) conversion was influenced by tissue types comprising different screen size fractions.  Chemical composition and SSF conversion data highlight the important differences found among feedstock particle size fractions.  Higher quality – greater total sugars and SSF conversion – miscanthus particle size fractions delivered greater theoretical ethanol yield.  These physical and biochemical properties influence the efficiency with which miscanthus biomass can be preprocessed and converted to bioproducts.