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Mechanical characteristics (stress and energy of tensile and shear modes) of selected biomass stems, such as big bluestem, bromegrass, and Barlow wheat were determined. A high capacity MTI-100K universal testing machine attached with standard tensile clamps and designed fabricated double-shear device were used for shear and tensile testing, respectively. Ultimate tensile stress and specific tensile energy values were statistically similar, while the shear stress and specific shear energy values were significantly different for the selected biomass stems. Ultimate shear stress and specific shear energy of bromegrass stems were significantly different (p<0.0001) from big bluestem and wheat stems, recording a largest value of 10.5 MPa and 27.0 kN/m, respectively. Stress values of shear failure mode were significantly smaller than tensile failure mode with evaluated shear to tensile stress ratios were 26.8%, 33.7%, 27.3%, while shear to tensile energy ratios were relatively constant as 54.3%, 55.1%, 54.0% for big bluestem, bromegrass, and wheat stems, respectively. Based on results, shear-dominant size reduction devices (e.g. knife mill, chipper) tend to be more energy efficient and this feature should be taken advantage of while designing size reduction devices.